Grind the oatmeal finely.
Mix whey protein isolate, cocoa powder, baking powder, and salt in a bowl.
Add egg, milk, and banana, and mix well.
Combine the oatmeal and other ingredients to make the batter.
Pour the batter into a brownie pan or muffin tin.
Bake in a preheated oven at 180°C for 20 minutes.