1. Preheat oven to 350 degrees. Lightly butter an 8-inch square metal baking pan and line with parchment paper, leaving a 2-inch overhang on all sides. In a large bowl, whisk together flour, cocoa powder, baking powder, and salt. In a medium saucepan, melt butter, sugar, and bittersweet chocolate over medium, stirring frequently, until smooth. Add to flour mixture and stir to combine. Stir in eggs and vanilla, then let cool to room temperature, 10 minutes.

2. Transfer batter to pan. Press 16 peppermint patties evenly into batter and, with a small offset spatula, evenly spread batter to completely cover patties.

3. Bake until cake is puffed and just set, about 35 minutes. Let cool on a wire rack, 25 minutes, then top with 16 peppermint patties. Let cool 25 minutes more, then decorate peppermint patties with candy eyes and chocolate chip ears. Refrigerate until chocolate is set, 5 minutes. Using parchment, lift cake from pan and cut into 16 squares. Decorate with icing pens.