Preheat your oven to 350°F (175°C). Line an 8x8-inch pan with parchment and lightly grease the sides.

In a large bowl, whisk the flour, oat flour, protein powder, sweetener, baking powder, baking soda, salt, and cinnamon until evenly combined. Breaking up clumps now helps the crumb stay tender.

In a separate bowl, whisk the eggs, Greek yogurt, applesauce, milk, vanilla, and melted oil until smooth. The mixture should be glossy and pourable.

Pour the wet mixture into the dry and stir gently with a spatula just until no dry streaks remain. Do not overmix—this keeps the cake soft.

In a small cup, stir together the cinnamon swirl ingredients: brown sugar, cinnamon, and melted butter.

Add about two-thirds of the batter to the prepared pan. Drizzle half the cinnamon swirl over the top, then use a knife or skewer to gently marble it. Add the remaining batter and drizzle the rest of the swirl. Marble again with gentle figure-eights.

Bake for 22–28 minutes, or until the center springs back and a toothpick comes out with a few moist crumbs. Protein bakes can dry quickly, so start checking at 20 minutes.

Cool in the pan for 10 minutes, then lift out using the parchment to finish cooling on a rack.

For the glaze, whisk powdered sweetener, Greek yogurt, vanilla, and a pinch of cinnamon. Add a splash of milk as needed until it’s thick but drizzleable. Spread or drizzle over the cooled cake.

Slice into squares. Enjoy warm for the full cinnamon roll effect or at room temperature for a denser, brownie-like bite.