Prep the pan and oven: Preheat your oven to 350°F (175°C). Lightly grease a 9x9-inch baking dish or line it with parchment.
Mix dry ingredients: In a large bowl, whisk together oat flour, protein powder, baking powder, baking soda, salt, and cinnamon.
Mix wet ingredients: In another bowl, whisk eggs, milk, Greek yogurt, maple syrup, vanilla, and melted coconut oil or butter until smooth.
Combine: Pour the wet mixture into the dry ingredients.
Stir gently until just combined. Do not overmix; a few small lumps are fine.
Add mix-ins: Fold in blueberries, chocolate chips, or your favorites. Reserve a handful to sprinkle on top for looks.
Pour and level: Transfer the batter to the baking dish and spread evenly.
Tap the dish once to release air bubbles. Sprinkle remaining mix-ins.
Bake: Bake for 22–28 minutes, until the center is set and a toothpick comes out clean or with a few moist crumbs.
Rest and slice: Let it cool for 10 minutes to set. Slice into squares and serve warm with a drizzle of maple syrup, a dollop of yogurt, or nut butter.