Preheat your oven to 350°F (175°C) and prepare a brownie tray or baking pan by lining it with parchment paper. Make sure the parchment slightly hangs over the edges of the pan so the brownies can be lifted out easily after baking. For even easier cleanup and smoother removal, lightly spray the parchment with cooking spray before adding the batter.

Add the banana to a large mixing bowl and mash it thoroughly using a fork until mostly smooth and creamy. A few tiny banana pieces are completely fine, but the smoother the banana becomes, the more evenly it will blend into the brownie batter. Using a ripe banana with brown spots works best because it adds more natural sweetness and moisture.

Pour the whole eggs, half of the egg whites, skimmed milk, and powdered sweetener into the bowl with the mashed banana. Whisk everything together until fully combined and smooth with no visible streaks of egg remaining. The mixture should look creamy and slightly pale after whisking. Mixing thoroughly at this stage helps create a more uniform brownie texture later.

Add the cocoa powder, vanilla whey protein powder, salt, baking soda, and baking powder into the wet mixture. Stir gently until a smooth chocolate batter forms. The batter will look thinner and wetter than traditional brownie batter, which is completely normal for high-protein brownies. Be careful not to overmix because excessive stirring can make the final brownies dense or rubbery.

In a separate clean mixing bowl, add the remaining egg whites and whip them using a hand mixer or stand mixer until firm peaks form. The egg whites should become fluffy, glossy, and thick enough to hold their shape when the beaters are lifted. This step is very important because the whipped egg whites create a lighter, softer brownie texture instead of a heavy protein-bar consistency.

Carefully fold the whipped egg whites into the chocolate batter using a silicone spatula. Use slow, gentle folding motions by scooping underneath the batter and lifting upward rather than aggressively stirring. This helps preserve the air inside the whipped egg whites, giving the brownies a softer and more tender texture after baking.

Pour the finished batter into the prepared baking tray and spread it evenly using a spatula so the brownies bake uniformly. Smooth the top gently to help create even thickness throughout the tray. If desired, sprinkle a few sugar-free chocolate chips or chopped dark chocolate pieces over the top before baking for extra richness.

Place the tray into the preheated oven and bake for 25–30 minutes, or until the edges look set and the center still feels slightly soft when lightly touched. Avoid overbaking because protein brownies continue firming up as they cool. Overbaking can quickly turn high-protein brownies dry or rubbery instead of soft and fudgy.

Remove the brownies from the oven and allow them to cool in the pan for at least 15–20 minutes before slicing. This cooling time is important because it helps the brownies set properly and makes them easier to cut into clean squares without falling apart. The texture also improves significantly as they cool.

Slice the brownies into 8 equal pieces and serve them as-is or topped with Greek yogurt, fresh berries, powdered sweetener, a drizzle of melted dark chocolate, or even a spoonful of powdered peanut butter sauce for extra protein. Enjoy warm for a softer brownie texture or chilled for a denser, fudgier bite.