Preheat oven to 375°F (190°C). Grease four ramekins or a large muffin tin with nonstick spray or oil.
Blend dry ingredients: In a blender, add rolled oats, protein powder, baking powder, and salt. Pulse into a fine flour.
Add wet ingredients to blender: Pour in eggs, milk, Greek yogurt, mashed banana, maple syrup, and vanilla. Blend until smooth.
Check consistency: Batter should be thick but pourable. Add 1 tbsp milk if too thick.
Fold in add-ins: Remove blender jar and gently fold in berries or chocolate chips with a spatula.
Portion batter: Pour batter evenly into the prepared ramekins, filling about ¾ full.
Bake: Place on a baking sheet and bake for 18–22 minutes until tops are golden and a toothpick comes out clean.
Cool & top: Let cool slightly, then add preferred toppings like yogurt, fruit, nut butter, and honey.