Preheat oven to 350 degrees. Prepare a 9x13-inch baking dish by spraying well with nonstick cooking spray.
Mix the wet ingredients (milk through flaxseed) together in a large bowl. Stir in the dry ingredients (oats, protein powder, powdered peanut butter, and spices) and mix until combined.
Spoon the oat batter into the prepared baking dish. Bake for 25-28 minutes or until the center is just set and the edges are a light golden brown.
Cool the baking dish on a cooling rack until room temperature. Drizzle with peanut butter (if your peanut butter is very thick, warm in the microwave for 30 seconds before drizzling.) Slice into 12 squares.
Serve warm, topped with banana slices or chopped nuts if desired (macros for additional toppings not included.)
This recipe is freezer friendly - freeze the squares in a freezer safe zip-top bag for up to 3 months. Defrost individual squares in the microwave for 30-60 seconds on medium power.
You could also try other flavors of Optimum Nutrition Gold Standard 100% Whey protein powder in this recipe. Chocolate, mocha cappuccino, chocolate peanut butter, or salted caramel would be particularly delicious!