Butter and flour 2 (9 x 5 x 3-inch) loaf pans and set aside. Preheat the oven to 350 degrees.
With an electric mixer cream the butter with 1/4 cup of the sugar and beat until it looks light and fluffy. Add the almond paste a bit at a time and continue to beat until the mixture looks light and fluffy. Add the yolks, 1 at a time, beating well after each addition, then beat in the vanilla.
Beat the egg whites with a pinch of salt until soft peaks form. Add the remaining sugar, a tablespoonful at a time, until the egg whites are stiff but not dry. Fold one quarter of the beaten whites into the butter/almond paste mixture, then fold that lightened mixture back into the remaining egg whites, sprinkling and adding in flour as you fold. Add melted chocolate, cutting through the batter with rubber spatula to marble the batter. Pour the batter into the prepared loaf pans and bake for 1 hour and 15 minutes or until a cake tester comes out clean when inserted in the middle of the cake; the cakes should be golden brown and pull away from sides of the pan. Let them set in the loaf pans for 15 minutes. Run a sharp knife around the edges and loosen the cakes onto a rack to cool completely.