Select a microwave-safe mug with at least a 12-ounce capacity to prevent overflow.
Whisk the protein powder, coconut flour, baking powder, cocoa powder, and sweetener in the mug until no clumps remain.
Add the egg and almond milk to the dry ingredients.
Mix thoroughly with a fork or small whisk until a smooth, uniform batter is achieved, ensuring you scrape the bottom edges of the mug.
Microwave on high power for 45 to 60 seconds; the cake is done when the top is set but still slightly springy.
Remove from the microwave and allow the cake to rest for 60 seconds to finish the internal steaming process before consuming.