Preheat oven to 350°F. Line an 8x8" baking pan with parchment paper.
In a large bowl, mix together mashed bananas, brown sugar, peanut butter, vinegar, milk, and vanilla extract.
Sift flour, cacao powder, baking soda, and baking powder on top of the banana mixture. Use a rubber spatula to gently fold together. Add an extra tablespoon or two of milk if the batter is very dry.
Mix until ingredients are just incorporated and no flour streaks remain. Be sure to scrape down the sides and the bottom of the bowl for dry flour. Add chocolate chips and gently fold a few times to disperse.
Pour batter into lined baking pan. Bake 25-30 minutes until a toothpick inserted in the center of the cake comes out clean or with just a few cooked crumbs.
Cool in the pan for 5 minutes, then transfer cake to a wire rack to finish cooling.
Protein Frosting
While the cake is baking, prepare the frosting. Drain tofu and place on a plate. Microwave 1 minute then drain excess water. Add to a food processor.
Add chocolate chips to a microwave-safe bowl. Microwave in 20 second intervals, stirring in between, until most of the chips are melted. Stir in peanut butter and microwave 15 seconds.
Add chocolate peanut butter mixture to food processor along with cocoa powder. Pulse until everything is well combined and mixture is smooth.
Transfer frosting to a bowl and chill in the fridge for 2-3 hours to thicken.
Once frosting has thickened and cake is cooled, spread frosting over cake. Serve immediately or cover and place in the fridge (the frosting will thicken a bit more).
Notes
Sugar: Feel free to use a different sweetener of choice such as maple syrup, agave, or coconut sugar. If using liquid sweetener, follow volume measurements (5 tbsp) instead of grams.