3/4cup (1 1/2 sticks) unsalted butter, plus more for coating foil
6ounces unsweetened chocolate, chopped
1 2/3cups sugar
1teaspoon vanilla extract
3/4cup all-purpose flour
3ounces bittersweet or semisweet chocolate, coarsely chopped
3/4cup coarsely chopped walnuts
FOR THE FROSTING
2cups heavy whipping cream
1/2cup fresh mint sprigs, plus more for garnish
1/2cup finely chopped bittersweet or semisweet chocolate
Position a rack in the center of the oven and preheat to 350 degrees F. Line a 9-inch square baking pan with 2-inch-high sides with foil. Butter the foil.
Stir 3/4 cup of the butter and the 6 ounces of unsweetened chocolate in a large bowl set over a medium saucepan of simmering water until melted and smooth. Cool until mixture is nearly room temperature.
Whisk in the sugar, vanilla and salt. Whisk in the eggs 1 at a time. Stir in the flour just until blended. Stir in the 3 ounces of bittersweet chocolate and the walnuts. Transfer the batter to the prepared pan.
Bake until a tester inserted into the center comes out with moist crumbs attached, about 30 minutes. Cool the brownies on a rack.
Combine 1/2 cup of the cream and 1/2 cup of the mint in a small saucepan and bring to a simmer over medium-high heat, muddling and stirring often. Place the finely chopped chocolate in a medium bowl. Set a strainer over the chocolate in the bowl. Pour the minted cream through the strainer and over the chocolate; discard the mint leaves. Stir until the chocolate melts and the mixture is well blended. Cool to room temperature.
Whip the remaining 1 1/2 cups of cream in a large bowl until soft peaks form. Gradually fold the chocolate mixture into the whipped cream until well blended but still light and airy.
Using the foil as an aid, lift the brownie from the pan. Remove the foil. Cut the brownies into squares. Spoon the frosting generously atop the brownies. Garnish with fresh mint and serve.