01 - Preheat oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper and lightly grease.
02 - In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, and salt.
03 - In a separate bowl, whisk melted butter, eggs, and vanilla extract. Incorporate this into the dry ingredients along with the milk, stirring until just combined.
04 - Pour batter into the prepared pan and spread evenly. Bake for 22–25 minutes or until a toothpick inserted into the center comes out clean. Allow to cool completely.
05 - Beat powdered sugar and softened butter until smooth. Add milk, peppermint extract, and food coloring (if using); beat until creamy and spreadable.
06 - Spread the peppermint cream evenly over the cooled cake base.
07 - Sprinkle crushed cookies, crushed peppermint candies, and chocolate chips over the cream layer, pressing lightly to adhere.
08 - Chill in the refrigerator for at least 30 minutes to set. Cut into 16 squares and serve.