Preheat oven to 175ºC (350°F) and fold a 20 cm (7.8 inches) cake mold with parchment paper
In a large mixing bowl, beat the egg together with the Greek yogurt until smooth.
Add oat flour, protein powder, cocoa powder and baking powder.
Start mixing, gradually adding the milk until you get a smooth cake batter - the amount of milk depends on the protein powder used (see notes*).
Transfer the protein batter to the prepared mold and bake in hot oven for 20 to 25 minutes. Let cool before serving.
