Preheat oven to 350°F (175°C) and grease an 8x8 inch baking pan with non-stick spray.
Whisk oat flour, whey protein powder, 1/4 cup cocoa powder, and baking powder in a large bowl.
Incorporate egg whites and applesauce into the dry ingredients until a uniform batter is achieved.
Transfer batter to the pan and bake for 22-25 minutes. Remove and allow to cool completely at room temperature.
Prepare the mousse layer by whisking Greek yogurt, casein protein powder, the remaining 1/4 cup cocoa powder, and almond milk until thick and smooth.
Cut the cooled protein cake into 1-inch cubes.
Layer half of the cake cubes at the base of four glass jars or one large trifle bowl.
Spread half of the chocolate protein mousse over the cake layer.
Repeat the process with the remaining cake cubes and the remaining mousse.
Top each serving with sugar-free whipped topping and sprinkle with chocolate chips.
Chill in the refrigerator for at least 30 minutes to set before serving.