Prepare the oven and pan: Preheat the oven to 350°F. Lightly grease a 9-inch pie pan or line the bottom with parchment paper
Make the brownie batter: Whisk the melted butter and granulated sugar together. Add the eggs and vanilla and mix until smooth. Stir in the cocoa powder, flour, and salt just until combined. Fold in the chocolate chips
Bake: Spread the brownie batter evenly into the prepared pan. Bake for 18 to 22 minutes, until the edges are set and the center is just firm. Do not overbake.
Cool completely: Place the brownie on a wire rack and let it cool completely before adding the mousse.
Prepare the cream cheese mixture: Beat the softened cream cheese until smooth. Add the powdered sugar, vanilla, and peppermint extract. Mix until creamy. Add a few drops of green food coloring if desired.
Whip the cream: In a separate bowl, beat the cold heavy cream until medium-stiff peaks form.
Make the mousse: Gently fold the whipped cream into the cream cheese mixture in batches. Fold in the mini chocolate chips.
Layer the pie: Spread the mint chip mousse evenly over the cooled brownie base.
Chill: Cover and refrigerate for at least 4 hours, or preferably overnight, until firm.
Add the topping: Beat the remaining heavy cream with powdered sugar and vanilla until fluffy. Spread or pipe it over the chilled pie and finish with chocolate chips or chocolate shavings
Serve: Slice with a sharp, warm knife for clean layers and serve cold.
