High protein brownies

Name: High protein brownies
Prep time: 10 min
Cook time: 20 min
Total time: 20 min
Yield: 12 pieces
Ingredients: butter; 80 g rolled oats; 100 g dark chocolate; 120 g full cream milk; 150 g Greek-style natural yoghurt; 80 g cocoa powder; 40 g cacao protein powder; 3 eggs; 2 tsp natural vanilla extract; 2 pinches salt; 1 ½ tsp baking powder; 2 tbsp walnut halves