Preheat the oven to 350°F / 180°C and lightly grease an 8×8‑inch or 9×9‑inch baking dish.
Mash the bananas in a large bowl, then whisk in soy milk, espresso, maple syrup, nut butter, vanilla and apple cider vinegar or lemon juice.
In a separate bowl, mix rolled oats, cocoa powder, protein powder if using, baking powder, baking soda and salt.
Pour the wet mixture into the dry ingredients and stir until fully combined.
Transfer the mixture to the prepared baking dish, smooth the top and bake for 35–40 minutes, until the centre is set and the top is lightly golden.
Allow the baked oats to cool completely in the dish.
For the yogurt layer, soften the nut butter slightly, stir in the vanilla and a spoonful or two of yogurt, then add the remaining yogurt, salt and protein powder if using, mixing until smooth; sweeten with maple syrup if desired.
Spread the yogurt mixture over the cooled baked oats.
Dust with cocoa powder just before serving.
Chill for at least 1 hour for best texture, then slice into squares.