Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius). Lightly grease an 8x8 inch baking dish with coconut oil or butter.

In a large mixing bowl, combine the rolled oats, baking powder, cinnamon, and salt. Stir until the cinnamon coats every grain of oat.

In a separate smaller bowl, whisk together the almond milk, eggs, maple syrup, and vanilla extract.

Pour the wet ingredients over the dry oats. Stir gently until everything is well incorporated. Let the mixture sit for about five minutes.

While the oats rest, mix the melted butter with brown sugar and extra cinnamon in a small ramekin.

Pour the oat mixture into the prepared baking dish. Spread it out evenly with a spatula.

Take spoonfuls of the cinnamon mixture and drop them onto the top of the oats. Use a knife or a toothpick to swirl the topping into the oats, creating marble patterns.

Place the dish in the oven and bake for 30 to 35 minutes, or until the edges are golden brown and the center is set but still slightly soft.

Let the oatmeal cool slightly before serving. Optionally, whisk together some powdered sugar and a splash of milk to drizzle over the top.