Preheat oven to 375° and grease baking pan with butter, set aside.
In a medium mixing bowl, whisk milk and egg together. Once lightly beaten, add maple syrup, vanilla, and melted butter. Whisk all wet ingredients together until mixed.
Add oatmeal, baking powder, and cinnamon to wet ingredients. Stir until evenly mixed. Let oatmeal rest for 5 minutes.
Once oatmeal has rested, gently fold in frozen blueberries. Pour oatmeal mixture into greased pan.
Bake at 375° for 28-30 minutes. The center should be fully set and will have a slight dome.