01 - Preheat oven to 350°F (175°C). Line a 9-inch square baking pan with parchment paper, ensuring an overhang for easy removal.
02 - In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, and salt.
03 - In a separate large bowl, beat the melted unsalted butter and granulated sugar until combined. Incorporate the large eggs and vanilla extract, mixing thoroughly.
04 - Gradually incorporate the dry ingredients into the wet mixture, alternating with the whole milk, until just combined to form the cake batter.
05 - Spread the batter evenly into the prepared baking pan. Bake for 22–25 minutes, or until a toothpick inserted into the center yields a few moist crumbs. Allow to cool completely within the pan.
06 - In a bowl, beat the softened cream cheese and softened unsalted butter until smooth. Gradually add the powdered sugar, beating until the mixture is fluffy. Stir in the peppermint extract, heavy cream, and green food coloring, if using.
07 - Spread the prepared peppermint cream evenly over the cooled cake base.
08 - In a bowl, combine the crushed chocolate sandwich cookies, mini chocolate chips, and crushed peppermint candy. Sprinkle this mixture evenly over the peppermint layer, pressing gently to adhere.
09 - Refrigerate for at least 2 hours before slicing into squares. Serve chilled.