Prepare the Oven and Pan
Set your oven to 375°F. Spray an 8×8-inch baking dish with non-stick cooking spray and set aside.
Mash the Bananas
Add the ripe bananas to a large mixing bowl and mash with a fork well before adding the other ingredients. You want about a cup of mashed banana.
Combine the Dry Ingredients
To the bowl, add the rolled oats, cocoa powder, chia seeds, espresso powder (if using), baking powder and kosher salt. With a spatula, mix the dry ingredients until well incorporated.
Add in the Wet Ingredients
Add in the almond milk, maple syrup, peanut butter, and vanilla extract. Mix until well integrated with the help of the spatula. Fold in half the chocolate chips or chopped chocolate.
Assemble and Bake
Pour the mixture into the prepared baking dish and spread it across evenly. Dot the rest of the chocolate on top. Bake for 30–35 minutes, until the oatmeal is puffed, set and the edges start to pull away from the dish.
Cool and Serve
Let the baked oatmeal cool on the counter for 10 minutes or so. Sprinkle with flaky sea salt, if desired. Serve warm for a creamy consistency, or allow it to cool further for a firmer texture that can be sliced easily.