2 1/2 cups unbleached flour
3/4 cup unsweetened cocoa
3 1/2 teaspoons baking powder
1/2 teaspoon salt
1 cup unsalted butter, at room temperature
1 1/2 cups packed light brown sugar
4 eggs, slightly beaten
2 teaspoons vanilla extract
1 cup walnuts, finely chopped
14 ounces bittersweet chocolate, melted
2/3 cups milk
1 1/2 cups confectioners' sugar