Choose a large mug (at least 12 oz) to prevent overflow. Lightly grease it with a dab of oil or nonstick spray.
Mix dry ingredients: In the mug, whisk together protein powder, cocoa powder, almond/oat flour, baking powder, and salt until no clumps remain.
Add wet ingredients: Stir in the egg, milk, vanilla, and sweetener.
If using, add coconut oil or nut butter. Mix until smooth and glossy.
Gooey center: Press chocolate chips or a chocolate square into the center. Don’t fully submerge; a little peek on top is fine.
Microwave: Cook on high for 40–60 seconds.
Start checking at 40 seconds. The edges should be set while the center looks slightly glossy.
Rest: Let it sit for 30–60 seconds. It will finish setting but stay fudgy.
Top and enjoy: Add a spoonful of yogurt, a drizzle of peanut butter, or a few berries if you like.