Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal, and lightly grease it.
Step 2: In a large bowl, whisk together the melted butter and granulated sugar until well combined.
Step 3: Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and 1 teaspoon of peppermint extract.
Step 4: In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
Step 6: Pour the brownie batter into the prepared baking pan and spread evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs, not wet batter.
Step 7: Remove from the oven and let the brownies cool completely in the pan on a wire rack before frosting. This is crucial for a clean cut.
Step 8: While brownies cool, prepare the mint frosting: In a medium bowl, beat the softened butter and powdered sugar until creamy. Add 2 tablespoons of milk, ½ teaspoon of peppermint extract, and green food coloring (if using) and beat until light and fluffy. Add more milk if needed to reach desired consistency.
Step 9: Once brownies are completely cool, spread the mint frosting evenly over the top.
Step 10: Melt the chocolate chips in a microwave-safe bowl in 30-second intervals, stirring until smooth. Drizzle the melted chocolate over the mint frosting.
Step 11: Let the frosting set slightly, then lift the brownies out of the pan using the parchment paper overhang and slice into 16 squares. Serve and enjoy!
Step 12: Store any leftover brownies in an airtight container at room temperature or in the refrigerator.