Heat oven to 180°C (350°F) and fold a 15 cm x 15 cm (7 inch x 7 inch) brownie pan with parchment paper.
First, in a large bowl mash the bananas and combine with yogurt, vanilla extract, lemon juice and a pinch of salt. If you want sweeter protein flapjacks, add maple syrup or honey to your likings. Stir until you get a smooth mixture.
Add rolled oats and protein powder to the wet mixture, while gradually pouring in the milk to consistency (see notes*). Once smooth, fold through the blueberries.
Transfer the protein dough to the prepared baking pan and top with more blueberries, if you desire.
Bake the protein flapjacks in hot oven for 20 to 30 minutes (baking time depends on the pan size). To make sure that they are ready, insert a toothpick in the center of your bars.