Preheat oven to 350°F (180°C). Grease or line a 9x13-inch baking dish with parchment paper.
In a large bowl, mix together the oats, protein powder (if using), baking powder, baking soda, cinnamon, and salt.
In a separate bowl, whisk together the soy milk, espresso (or coffee), maple syrup, vanilla, and cashew or almond butter until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until combined.
Add the apple cider vinegar and stir briefly to activate.
Pour the batter into the prepared baking dish and smooth the top.
Bake for 35 to 40 minutes, or until the center is set and the top is golden brown.
Let cool completely in the pan for clean slicing.
In a medium bowl, mix the yogurt, protein powder (if using), and maple syrup until smooth.
Once the oats are cool, spread the yogurt mixture evenly over the top.
Dust with cocoa powder just before serving.