In a large bowl, whisk together all ingredients until well combined.
Transfer mix to an airtight container or zip-top bag. Label with preparation instructions. Store in a cool, dry place for up to 2 months. Avoid humidity and keep container of dry mug cake mix tightly closed.
To make one serving of mug cake: Combine 3 T cake mix with 2 T water in a microwave-safe mug. Stir until the batter looks even and a bit foamy.
Microwave on high for 1 minute, or until cake is puffed and set. Don't overcook; you may see a bit of moisture but it will finish cooking as it rests.
Top with frosting, powdered sugar, Nutella, chocolate sauce, or whipped cream. Enjoy warm and directly out of the mug.