1 If the slices of cake are a little dry, spread a little butter or margarine on each slice. 2  For heated fruit cake as a dessert, place the slices on a microwave-safe dish and heat for 20 seconds for one slice or a minute for four slices. 3  Serve with ice cream, whipped cream, or custard.d. 2. Keep the fruit cake in the refrigerator. This will make it much easier to cut. If you do this, after cutting leave the cake slices to reach room temperature before serving. 3. Serve the slices. 1  If the slices of cake are a little dry, spread a little butter or margarine on each slice.