How to Make It
Preheat the oven to 325°. Line the bottom of a 9-by-13-inch metal baking pan with parchment paper. In a small bowl, whisk the sugar, with the flour, baking powder and salt.
In a saucepan, stir the chocolate and butter over very low heat until melted; let cool slightly. In a large bowl, whisk the eggs with the sour cream and vanilla. Whisk in the dry ingredients, then the melted chocolate mixture. Transfer 2 tablespoons of the batter to a small bowl and stir in the peanut butter.
Scrape the batter into the prepared baking pan and dollop the peanut butter over the top. Swirl in the peanut butter, but don't overmix. Bake the brownies for 45 minutes, or until risen and the top is lightly cracked and glossy; the brownies will still be jiggly. Transfer the pan to a rack to cool, then refrigerate until the brownies are chilled, at least 2 hours. Cut into 18 rectangles.
In a large skillet, melt the dulce de leche over moderate heat. Add the bananas and cook, turning occasionally, until heated through, about 3 minutes. Set 8 brownies on individual plates and top with the bananas and dulce de leche. Spoon the ice cream and whipped cream alongside, top with the crushed peanut brittle and serve.