Yes, you can! If you don't have a microwave, find a very small cake pan or use a large oven-safe ramekin that is at least 250 mL or 1 cup. Some mugs will go in the oven, like enamelware mugs and ceramic mugs, but you'll have to check with the manufacturer. Bake the mug cake in a preheated oven set to 325ºF or even 350 ºF. It will take under 20 minutes. You can use a conventional oven or a toaster oven. Remember to check if the cake is done baking before taking it out! Use a cake tester or an instant read thermometer.