Preheat and prep: Heat the oven to 350°F (175°C). Lightly grease an 8x8-inch or 9x9-inch baking pan and line with parchment for easy removal.
Make oat flour: If using rolled oats, blend them in a blender until they become a fine flour.
Measure after blending to ensure you have 1 1/2 cups.
Mix dry ingredients: In a large bowl, whisk oat flour, protein powder, baking powder, baking soda, salt, and cinnamon until evenly combined.
Whisk wet ingredients: In a separate bowl, whisk eggs, Greek yogurt, milk, maple syrup, and vanilla until smooth. Stir in lemon zest if using.
Combine: Pour the wet mixture into the dry ingredients. Stir with a spatula until just combined.
The batter should be thick but scoopable—add a splash more milk if it seems too stiff.
Fold in blueberries: Toss blueberries with 1 teaspoon of oat flour (optional, helps prevent sinking), then gently fold them into the batter.
Bake: Spread batter evenly in the pan. Bake for 22–30 minutes, or until the top is lightly golden and a toothpick inserted in the center comes out mostly clean with a few moist crumbs.
Cool and slice: Let it cool in the pan for 10–15 minutes. Lift out using the parchment, slice into 9 or 12 squares, and serve warm.
Serve it up: Top with a little maple syrup, a dollop of Greek yogurt, peanut butter or almond butter, or a sprinkle of extra berries.