Preheat the oven to 325 degrees. Use a little of the butter to grease a 10-inch cake pan.

Place the raisins or currants in a bowl, add the warm brandy or whiskey, and set aside. Place the remaining butter and the chocolate in a saucepan and melt them over low heat. Stir. (The butter and chocolate can also be melted in a microwave oven for about 1 minute.)

In a large bowl, beat the eggs and sugar until well blended, pale, and creamy, about 4 minutes. Sift the potato starch and fold it in. Stir in the chocolate and butter mixture, then stir in the raisins or currants and brandy and the vanilla.

Pour the batter into the pan and bake for about 40 minutes, until the cake has firmed up on top but a tester does not come out perfectly clean. It should be slightly underdone. Remove from the oven and cool to room temperature, then cover and refrigerate for at least 3 hours or overnight.

Remove the cake from the refrigerator and unmold it 1 hour before serving.