Preheat the oven to 400°. Spread the raspberries in a gratin dish. Break the cake into chunks and process it to crumbs in a food processor; you should have about 2 cups. Add the butter and brown sugar, and process until the crumbs appear mealy. Sprinkle the crumbs on top of the berries.
Bake in the oven until nicely browned on top 20 to 25 minutes if using fresh berries, 25 to 30 minutes if using frozen. Keep warm until serving time or let cool and then reheat. Garnish each serving with a spoonful of sour cream or crème fraîche.
This recipe appears in Jacques Pépin's The Short-Cut Cook, published in paperback in 2001 by William Morrow, an imprint of HarperCollins Publishers.