Preheat oven to 350°F (180°C). Grease or line an 11x8-inch (28x20 cm) baking pan with parchment paper.
In a large bowl, whisk together the oats, cocoa powder, protein powder (if using), baking powder, baking soda, and salt.
In a medium bowl, whisk the soy milk, maple syrup, vanilla, nut butter, vinegar (or lemon juice), and espresso until smooth.
Pour the wet ingredients into the dry and stir until just combined.
Fold in the chocolate chips. Transfer the batter to the prepared pan and smooth the top.
Bake for 30–35 minutes, until the center is set and the edges pull slightly away from the pan.
Let cool completely in the pan before frosting.
For the frosting, stir together the yogurt, protein powder (or sub with cocoa), maple syrup, and salt until creamy. Spread evenly over the cooled bake. Chill before slicing into squares.
