8 tablespoons (1 stick) unsalted butter, cut into small pieces, plus more for pan
2 ounces good-quality unsweetened chocolate, coarsely chopped
4 ounces good-quality semisweet chocolate, coarsely chopped
2/3 cup all-purpose flour
1/2 teaspoon baking powder
1/4 teaspoon salt
3/4 cup granulated sugar
3 large eggs
2 teaspoons pure vanilla extract
4 tablespoons unsalted butter, melted
1/2 cup confectioners' sugar
3/4 cup smooth peanut butter
1/4 teaspoon salt
1/2 teaspoon pure vanilla extract