Mint Chocolate Sugar Cookie Bars
Bring your St. Patrick’s Day to life with this fun treat!
These Mint Chocolate Sugar Cookie Bars are rich, fudgy, and topped with a smooth peppermint-infused green frosting, perfect for St. Patrick’s Day! They slice beautifully and are sure to be a hit at any gathering.
St. Patrick’s Day in our house meant the kitchen smelled like sugar, butter, and just a hint of mint as I helped bake festive treats for my family. My grandma’s old mixing bowls clattered on the counter while I measured flour and sneaked a little extra vanilla into the batter because a touch more vanilla always makes things taste better! The oven warmed the whole kitchen as chocolate cookie bars baked to fudgy perfection, and I watched through the oven window, imagining the moment I’d spread the frosting—green, of course, as bright as fresh clover. The best part was always the sprinkles, which I scattered generously, pretending each one was a tiny piece of good luck. My family would gather around, waiting for the first bite, and I’d beam as they declared them the best yet. Those simple moments—flour-dusted hands, laughter, and the joy of sharing something homemade—made every St. Patrick’s Day feel like a little bit of magic. While I have only 10% Irish ancestry, that never stopped the fun! I always say that I’m 100% of all my piece parts.
Around the world, baking plays a big role in St. Patrick’s Day festivities, especially in Irish-American households. Traditional Irish soda bread, made with baking soda instead of yeast, is a classic, while modern celebrations include festive green desserts, mint-flavored treats, and chocolate delights. From shamrock-shaped cookies to peppermint-frosted cakes, baking brings people together, turning the holiday into a time of warmth, nostalgia, and sweet homemade traditions.
Note: No adjustments were necessary for these cookie bars at 7,000 feet.
On Measuring Ingredients: My preferred method of measuring is weight by grams. Baking by volume rather than weight can lead to varied and unexpected results. Flour is especially problematic when measured by volume (sifted, not sifted, spooned, not spooned, fluffed or not … you get the point). Grams are always the same; they are your friend as a baker. King Arthur Baking Company’s Ingredients Weight Chart is helpful for converting recipe weights.
On Altitude Adjustments: Before measuring ingredients, preheating your oven, or setting your timer; it’s important to consider any necessary adjustments for baking at higher altitudes, which is generally considered to be 3,500ft and above. Most recipes are written for sea level up to 3,500ft. Take at look at our baking at High-Altitude Baking Adjustments page for guidance.
Recipe
Mint Chocolate Sugar Cookie Bars
Makes: One 9x13-inch (23x33 cm) pan
For the Sugar Cookie Base:
½ cup (113g) butter, softened – Adds richness, moisture, and a tender texture.
½ cup (110g) brown sugar – Provides sweetness and a hint of molasses for depth of flavor.
½ cup (100g) granulated sugar – Sweetens and helps create a soft, slightly chewy texture.
1 large egg (50g) – Binds the ingredients together and adds structure.
2 teaspoons vanilla extract – Enhances flavor and adds warmth.
¼ cup (50g) sour cream or plain Greek yogurt – Adds moisture and tenderness, preventing dryness.
2 cups (250g) all-purpose gluten-free flour blend or regular all-purpose flour – Forms the structure of the bars.
½ cup (56g) unsweetened cocoa powder – Gives a deep, rich chocolate flavor.
½ teaspoon baking powder – Helps the bars rise slightly for a soft texture.
½ teaspoon baking soda – Reacts with the sour cream for additional leavening.
½ teaspoon salt – Balances the sweetness and enhances the chocolate flavor.
For the Peppermint Frosting:
½ cup (113g) butter, softened – Creates a creamy, smooth base for the frosting.
¼ cup (61g) milk – Helps adjust the frosting’s consistency for easy spreading.
2 teaspoons vanilla extract – Adds warmth and rounds out the peppermint flavor.
½ teaspoon peppermint extract – Provides the cool, refreshing minty flavor.
3-4 cups (360-480g) powdered sugar – Sweetens the frosting while creating a fluffy, stable texture.
Green gel food coloring – Gives the frosting a festive St. Patrick’s Day color.
⅓ cup (about 50g) sprinkles (optional but why not?) – Adds decoration and a fun crunch on top
Step 1: Prepare the Pan and Oven
Preheat the oven to 350°F (175°C) for conventional or 325°F (160°C) for convection. Grease and line a 9x13-inch (23x33 cm) baking pan with parchment paper, leaving an overhang for easy removal.
Step 2: Make the Chocolate Sugar Cookie Bar Base
In a large mixing bowl, beat the butter, brown sugar, and granulated sugar together for about 2 minutes, until light and fluffy. Mix in the egg, vanilla, and sour cream (or Greek yogurt) until smooth.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing just until combined. The dough will be thick. Press it evenly into the prepared pan.
Bake for 15-18 minutes in a conventional oven (13-16 minutes in convection) or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs. Let cool completely in the pan before frosting.
Step 3: Make the Peppermint Frosting
In a large mixing bowl, beat the butter until smooth and creamy. Mix in the milk, vanilla, and peppermint extract. Gradually add the powdered sugar, 1 cup at a time, beating until smooth and fluffy. Adjust the consistency by adding more powdered sugar if needed. Add a small amount of green gel food coloring at a time until the frosting reaches your desired shade.
Step 4: Assemble and Decorate
Spread the frosting evenly over the cooled chocolate cookie bars. Add sprinkles for a festive touch if desired. Lift the bars out of the pan using the parchment paper overhang, then slice into squares and serve.
Pro Tips for Perfect Cookie Bars
Baking time: Keep an eye on the bars—since every oven is different, start checking for doneness at the lower time range.
Chilling for clean cuts: For neater slices, chill the bars in the refrigerator for 30 minutes before cutting.
Food coloring: Use gel food coloring instead of liquid to maintain a smooth frosting consistency.
These bars are perfect for St. Patrick’s Day, bake sales, or anytime you crave a chocolate-mint treat! Let me know how they turn out!
Equipment (Disclosure: Items below include affiliate links with Amazon USA. You pay the regular price, and I get a small commission.) I only recommend products that I own myself and know to be excellent quality. I often get asked on social media sites about my kitchen tools, so here are most of them used to make this recipe. The full list of our recommended kitchen equipment can be found here.
9x13-inch baking tin (This is the perfect size for a variety of brownies, bars, and traybakes.)
Small Baking Palette Knives (These are great for leveling and teasing batters into pan corners.)
Mixing bowls (I like this oven-proof glass set of 10 for mixing and portioning out ingredients.)
Mesh strainers/sifters/sieves (Sifting dry ingredients is important.)
Stand mixer (This dough can be mixed by hand, but a stand mixer ensures consistency.)
Glass mixing bowl for KitchenAid stand mixer (See all your ingredients as they mix.)
Cordless electric hand mixer (This is a handy mixer for these smaller bakes.)
Kitchen scale (Ingredients should be weighed rather than relying on volume.)
Oven thermometer (All ovens need calibration, an in-oven thermometer is essential.)
Silicon oven mitts (I’ve burned my hands too many times with cloth mitts.)
Thanks for popping into the Mountaintop Kitchen today!