Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius) and grease a 9x9 inch baking dish.

In a large mixing bowl, whisk together the rolled oats, brown sugar, 2 teaspoons of cinnamon, baking powder, and salt.

In a medium bowl, combine the milk, eggs, melted butter, and vanilla extract until homogeneous.

Incorporate the wet mixture into the dry ingredients, stirring until the oats are thoroughly hydrated.

Transfer the oat mixture into the prepared baking dish, leveling the surface with a spatula.

In a small vessel, combine the granulated sugar and 1 tablespoon of cinnamon; sprinkle this mixture evenly over the top of the oats.

Bake for 30 to 35 minutes, or until the center is firm to the touch and the topping has crystallized.

Allow the dish to rest for 5 minutes at room temperature before slicing and serving.