Preheat your oven to 350°F and lightly grease your mini ramekin with a bit of cooking spray or oil. This step keeps your baked oats from sticking once they're done and ready to be scooped out or served straight from the dish.
Add your rolled oats to a spice grinder or blender and pulse until they turn into a fine, flour-like texture. If you already have oat flour on hand, you can skip this step entirely and use that instead.
In a mixing bowl, whisk together the oat flour, baking powder, cocoa powder, and salt. Then pour in the milk, maple syrup, and vanilla extract, stirring until you get a smooth, thick batter with no lumps.
Pour about two-thirds of the batter into your prepared ramekin. Spoon the peanut butter and chocolate chunks right into the center, then cover with the remaining batter so the filling is tucked inside.
Bake for 20 to 25 minutes, until the top looks set and a toothpick inserted near the edge comes out clean. Let it cool for a couple of minutes, then finish with a sprinkle of flaky sea salt before digging in.