Mashing berries with sugar releases their juices and intensifies their flavor, creating a rustic puree for desserts.
Place bottom layer of cake on a large piece of foil or in a loaf pan of equal size. Sprinkle half the raspberries over cake, spread evenly with half the ice cream, and top with another cake layer. Repeat with remaining raspberries, ice cream, and cake.
Wrap with foil and freeze at least 4 hours (or up to overnight). Remove cake from freezer about 30 minutes before serving. Serve dusted with confectioners' sugar.