01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing overhang for easy removal.
02 - In a medium saucepan, melt butter over low heat. Remove from heat and whisk in sugar, eggs, and vanilla until smooth. Stir in cocoa powder, flour, salt, and baking powder until just combined.
03 - Spread batter evenly in prepared pan. Bake for 25 to 30 minutes, until a toothpick inserted in center comes out with few moist crumbs. Cool completely in pan.
04 - In a medium bowl, beat softened butter until creamy. Gradually add powdered sugar, mixing until smooth. Add cream, peppermint extract, and green food coloring. Beat until light and fluffy. Spread evenly over cooled brownies.
05 - Refrigerate brownies for at least 30 minutes.
06 - Place chocolate and butter in a heatproof bowl. Heat cream in a small saucepan until just simmering. Pour cream over chocolate and let sit for 2 minutes. Stir until smooth and cool slightly.
07 - Pour and spread ganache evenly over mint layer. Refrigerate for at least 30 minutes until ganache is set. Lift from pan, cut into 16 squares, and serve.