If you're reheating a slice of cake that has already been cut, the best way to do so is in the microwave. Simply wrap the slice in a damp paper towel and heat for 30-45 seconds, or until warmed through. If you're reheating an entire cake, preheat your oven to 350 °F and place the cake on a baking sheet. Heat for 10-15 minutes, or until warmed through.