Choose your mug. Use a large, microwave-safe mug (at least 12 oz) to prevent overflow.
Mix the wet base. Add mashed banana or Greek yogurt to the mug.
Stir in the nut butter until smooth.
Add protein powder. Sprinkle it in and mix well. If the batter is very thick or dry, add 1–2 teaspoons of milk or water. You want a thick, spoonable batter.
Optional lift. For a fluffier cake, stir in a small pinch (about 1/8 teaspoon) of baking powder.
Microwave. Heat on high for 45–60 seconds.
Check the center. If it’s still wet, microwave in 10–15 second bursts until the top looks set and springs back slightly.
Rest. Let it sit for 1 minute to finish cooking and firm up.
Add toppings. Enjoy as is, or top with a few chocolate chips, a drizzle of nut butter, a spoon of Greek yogurt, or fresh berries.