Prep the pan: Heat oven to 350°F (175°C). Grease an 8x8-inch baking dish or line with parchment.
Mix the wet ingredients: In a large bowl, whisk Greek yogurt, eggs, applesauce, maple syrup, and vanilla until smooth.
Add the dry ingredients: Stir in protein powder, oat flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
Fold in 2 tablespoons melted oil or butter. The batter will be thick but spreadable.
Make the swirl: In a small bowl, combine melted oil or butter, brown sugar, and cinnamon.
Assemble: Spread half the batter in the dish. Drizzle half the swirl over it.
Add remaining batter, then the rest of the swirl. Use a knife to gently marble it. Sprinkle chocolate chips or nuts if using.
Bake: Bake 22–30 minutes, until the center is set and a toothpick comes out mostly clean.
Don’t overbake or it will dry out.
Glaze: Whisk powdered sugar, milk, and vanilla until pourable. Let the bake cool 10–15 minutes, then drizzle over the top.
Slice and serve: Cut into 9 squares. Enjoy warm or at room temperature.