Prepare the Oven and Pan
Preheat your oven to 375°F. Lightly coat an 8x8-inch baking dish with non-stick cooking spray and set it aside.
Mash the Bananas
In a large mixing bowl, mash the ripe bananas thoroughly with a fork until smooth. Aim for approximately one cup of mashed banana.
Combine the Dry Ingredients
Add the rolled oats, cocoa powder, chia seeds, espresso powder (if using), baking powder, and kosher salt to the bowl. Mix the dry ingredients with a spatula until well incorporated.
Incorporate the Wet Ingredients
Pour in the almond milk, maple syrup, peanut butter, and vanilla extract. Stir the mixture until it is evenly combined. Fold in half of the chocolate chips or chopped chocolate.
Assemble and Bake
Transfer the mixture to the prepared baking dish, spreading it evenly. Sprinkle the remaining chocolate on top. Bake for 30–35 minutes, or until the oatmeal is puffed, set, and edges begin to pull away from the dish.
Cool and Serve
Allow the baked oatmeal to cool on the countertop for about 10 minutes. Garnish with flaky sea salt, if desired. Serve warm for a creamy texture, or let it cool further for a firmer consistency that can be easily sliced.