Preheat the oven to 175°C conventional or 160°C fan-assisted. Lightly grease and line a 20 × 20 cm or 23 × 23 cm baking dish.
Mash the bananas in a large bowl until smooth.
Whisk in the soy milk, cooled espresso, maple syrup, nut butter, vanilla, and apple cider vinegar.
In a separate bowl, mix the rolled oats, cocoa powder, optional protein powder, baking powder, baking soda, and salt.
Pour the wet ingredients into the dry ingredients. Stir until fully combined and leave the mixture to rest for 5 minutes.
Transfer the batter to the prepared dish and smooth the surface.
Bake for 35–40 minutes, or until the edges and center are set. A skewer may have a few moist crumbs but should not have liquid batter.
Cool the baked oats completely.
To prepare the topping, mix the softened nut butter with the vanilla and two spoonfuls of yogurt until smooth.
Add the remaining yogurt, salt, and optional protein powder. Stir until creamy. Sweeten with maple syrup to taste.
Spread the yogurt mixture over the cooled oat base.
Refrigerate for at least 1 hour, preferably overnight.
Dust with cocoa powder immediately before slicing and serving.