Prep the pan: Heat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease lightly.
Make oat flour (optional): Pulse 1 cup of the rolled oats in a blender until sandy.
This improves texture. Keep 1/2 cup oats whole for chew.
Whisk dry ingredients: In a large bowl, combine 1 cup oat flour, 3/4 cup white whole wheat flour, 1/2 cup protein powder, 2 tablespoons ground flaxseed, 1 1/2 teaspoons baking powder, 1/2 teaspoon baking soda, 1 teaspoon cinnamon, 1/4 teaspoon salt, and the remaining 1/2 cup whole oats.
Whisk wet ingredients: In a separate bowl, mix 1 cup plain Greek yogurt, 1/2 cup unsweetened applesauce, 2 large eggs, 1/3 cup maple syrup, 1/4 cup neutral oil, 1 teaspoon vanilla, and 1/3 cup milk.
Combine gently: Pour the wet mixture into the dry and fold with a spatula until just combined. The batter will be thick.
If it seems dry, add 1–2 tablespoons more milk.
Add mix-ins: Fold in up to 1 cup total of add-ins, such as 3/4 cup blueberries and 1/4 cup chopped walnuts. Don’t overmix.
Fill the cups: Divide batter evenly among the 12 cups. They should be nearly full for domed tops.
Bake: Bake 18–22 minutes, until the tops spring back and a toothpick comes out clean or with a few moist crumbs.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a rack.
Cooling helps them set and prevents soggy bottoms.
Enjoy: Serve warm or at room temperature. For extra protein, smear with Greek yogurt or nut butter.