Preheat your oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper.
Stir together the melted 1 cup unsalted butter, 2 cups granulated sugar, 4 large eggs, and 1 tsp vanilla extract until well combined.
1 cup unsalted butter, 2 cups granulated sugar, 4 large eggs, 1 tsp vanilla extract
In a separate bowl, whisk together the 1 cup all-purpose flour, 1/2 cup unsweetened cocoa powder, 1/2 tsp salt, and 1/2 tsp baking powder. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
1 cup all-purpose flour, 1/2 cup unsweetened cocoa powder, 1/2 tsp salt, 1/2 tsp baking powder
Pour the batter into the prepared pan and spread evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Allow the brownies to cool completely in the pan until no longer warm to the touch.
In a large bowl, beat the softened 1/2 cup unsalted butter with an electric mixer until creamy. Gradually add the 2 cups powdered sugar, 2 tbsp heavy cream, and 1 tsp peppermint extract, beating until smooth and fluffy.
1/2 cup unsalted butter, 2 cups powdered sugar, 2 tbsp heavy cream, 1 tsp peppermint extract
Spread the mint filling over the cooled brownies in an even layer. Chill in the fridge for at least 30 minutes.
Place the 1 cup semisweet chocolate chips in a heatproof bowl. In a small saucepan, heat the 1/2 cup heavy cream over medium heat until it just begins to simmer.
1 cup semisweet chocolate chips, 1/2 cup heavy cream
Pour the hot cream over the chocolate chips and let sit for 2-3 minutes. Then, stir until the mixture is smooth and glossy.
Pour the ganache over the chilled mint layer, spreading it evenly with a spatula. Refrigerate again until the ganache is set, about 1 hour.
Once the ganache is set, cut the brownies into squares using a sharp knife. For cleaner cuts, wipe the knife with a warm, damp cloth between slices.