Chocolate Prune Cake

Name: Chocolate-Prune Cake
Ingredients: 6 ounces (170 g) pitted prunes, diced in small pieces; 1/3 cup (80 ml) rum, or another liquor (see headnote); 1 tablespoon sugar; 12 ounces (340 g) bittersweet or semisweet chocolate, coarsely chopped; 12 tablespoons (6 ounces, 170 g) unsalted butter, cubed; 6 large eggs, separated; large pinch of salt; 3 tablespoons sugar; Additional soft butter and flour, or cocoa powder, for preparing the pan