For the Brownies:
1¼ cups all-purpose flour
1 teaspoon salt
2 tablespoons dark unsweetened cocoa powder
11 ounces dark chocolate, coarsely chopped
1 cup (8 ounces) unsalted butter, cut into 1-inch pieces
1 teaspoon instant espresso powder
1½ cups granulated sugar
½ cup packed light brown sugar
5 eggs, at room temperature
2 teaspoons vanilla extract

For the Peanut Butter Swirl:
½ cup (8 tablespoons) unsalted butter, melted
1 cup powdered sugar
1½ cups creamy peanut butter
½ teaspoon salt
1 teaspoon vanilla extract