If baking, preheat the oven to 350°F (175°C). Grease a mug or one to two ramekins well.
In the mug or a small bowl, whisk together the cocoa powder, flour, chocolate protein powder, salt, baking powder, and the first 2 tablespoons of sugar until evenly combined.
Add the oil, water or milk, vanilla extract, and the remaining 4 teaspoons sugar (or a pinch of stevia) to the dry ingredients; stir until a smooth batter forms.
If you mixed in a bowl, transfer the batter to the prepared mug or ramekins.
For the microwave: cook on high for about 2 minutes, or until the cake is set and springs back slightly when touched (microwave times vary).
For the oven: bake at 350°F (175°C) for about 12 minutes, or until a toothpick comes out clean and the cake is springy.
Serve warm straight from the mug or loosen the edges with a knife after cooling and remove if desired; top as you like.