01 - Preheat oven to 350°F. Lightly grease an 8x8-inch baking dish with butter.
02 - In a large mixing bowl, blend together rolled oats, baking powder, salt, and 1 1/2 teaspoons ground cinnamon.
03 - In a separate bowl, whisk eggs with maple syrup, milk, melted butter, and vanilla extract until thoroughly combined.
04 - Fold the wet mixture into the dry ingredients until evenly moistened. Gently incorporate nuts or fruit if desired.
05 - Spread oatmeal mixture evenly in the prepared baking dish.
06 - In a small bowl, stir together brown sugar, 1 tablespoon ground cinnamon, and 2 tablespoons melted butter until combined.
07 - Drizzle cinnamon swirl evenly over the oatmeal batter. Use a knife to gently swirl it through the top layer.
08 - Bake for 35 to 40 minutes or until the center is set and the top is golden brown.
09 - Allow baked oatmeal to cool for 10 minutes before slicing. Serve warm.