Stir together all the dry ingredients for the cake (oats, flour, stevia powder, baking powder and ground cinnamon) in a large mug or ramekin with a fork.
Add the yogurt and stir thoroughly to make sure there are no dry pockets hiding anywhere.
Place the mug or ramekin in the microwave and cook on full power for 80-90 seconds, until the cake has puffed up and it's no longer wet on top.
Remove the cake from the microwave and top with the yogurt and a sprinkling of ground cinnamon before eating hot.