Microwave the cake for several minutes to cook the inside of the cake. Microwaving times vary depending on the size of the cake. If the cake is still warm, put it back in the oven. It is also possible to cover a cool cake and bake it at a low temperature until the inside is fully cooked. However, this may overcook the outside of the cake.
Most cakes are difficult to fix if they are already covered with icing. Because icing melts when warm, remember to remove it before attempting to fix the cake. Try microwaving a standard 9-inch cake for 3 minutes. Baking the cake is risky, but wrapping the cake in aluminum foil or using a covered baking dish ensures that the outside does not overcook. Collapsed cake is difficult to fix, but it is possible to bake or microwave the cake and then serve it in slices rather than as a whole cake. Finally, if re-baking the cake is not an option, it is also possible to cut out the undercooked center to serve the cake as-is or to serve the cake in muffin tins or small glasses. Serving small chunks of cake in glasses looks intentional with a small amount of creativity.