Prep the pan and oven: Preheat oven to 350°F (175°C).
Grease a 9x9-inch or 8x10-inch baking dish.
Blend or mix the base: Add oats to a blender and pulse into a flour. If you prefer texture, leave some oats whole. In a large bowl, whisk eggs, yogurt, milk, maple syrup, and vanilla until smooth.
Add dry ingredients: Stir in oat flour, protein powder, baking powder, baking soda, cinnamon, and salt.
Mix until just combined. The batter should be pourable; add a splash of milk if too thick.
Fold in extras: Gently fold in any mix-ins like berries or chocolate chips.
Pour and level: Pour batter into the prepared dish and smooth the top with a spatula.
Bake: Bake for 22–28 minutes, or until set in the center and lightly golden on top. A toothpick should come out mostly clean.
Cool and slice: Let it cool for 10 minutes.
Slice into 6–9 pieces, depending on your serving size.
Serve: Top with fresh fruit, a spoonful of yogurt, or a light drizzle of maple syrup. For extra protein, add a smear of almond or peanut butter.