For the brownie base:
3/4 cups flour
1/2 teaspoon salt
1 tablespoon dark unsweetened cocoa powder (like Valrhona)
5 ounces good quality dark chocolate (60 to 72%), coarsely chopped
1/2 cup (1 stick) butter, cut into 1 inch cubes
3/4 cups sugar
1/4 cup firmly packed light brown sugar
3 large eggs, at room temperature
1 teaspoons vanilla extract

For the buttercream:
3/4 cups sugar
2 tablespoons flour
3/4 cup milk
2 tablespoons heavy cream
1 1/2 sticks (3/4 cup) butter, softened but still cool, cut into small cubes
3 tablespoons creme de menthe
1 teaspoon peppermint extract

For the chocolate glaze:
6 ounces good-quality dark chocolate (60 to 72%), coarsely chopped
1 teaspoon light corn syrup
1/2 cup (1 stick) unsalted butter, softened, cut into cubes