Brownie (bottom layer):
1 1/2 sticks (3/4 cup) unsalted butter
10 1/2 oz fine-quality bittersweet chocolate (not extra-bitter or unsweetened and no more than 60% cacao if marked), finely chopped (I used Guittard semi-sweet chips)
1 1/2 cups packed light brown sugar
3 large eggs, lightly beaten
1 1/4 teaspoons vanilla
3/4 cup all-purpose flour
1/4 cup plus 2 tablespoons unsweetened Dutch-process cocoa powder
3/4 teaspoon salt

Mint ganache (middle layer):
1/2 cup heavy cream
10 oz fine-quality white chocolate, chopped (I used Ghirardelli white chocolate chips)
2 tablespoons green crème de menthe
1 teaspoon peppermint extract

Chocolate ganache (top layer):
1 cup heavy cream
10 oz fine-quality bittersweet chocolate (not unsweetened), finely chopped (I used Guittard semi-sweet chips)