01 - Preheat the oven to 375°F. Lightly grease a 12-cup muffin tin or line with silicone baking liners.
02 - In a large mixing bowl, whisk together the rolled oats, baking powder, ground cinnamon, and salt until evenly distributed.
03 - In a separate bowl, whisk together the milk, egg, maple syrup or honey, melted coconut oil or butter, and vanilla extract.
04 - Pour the wet mixture into the dry ingredients and stir gently until just combined.
05 - In a small bowl, mix the brown sugar, ground cinnamon, and melted butter to create a thick swirl mixture.
06 - Divide the oatmeal batter evenly among the muffin tin cavities, filling each approximately three-quarters full.
07 - Drop one teaspoon of the cinnamon swirl mixture into the center of each cup and use a toothpick or skewer to gently swirl through the batter.
08 - Place the muffin tin in the oven and bake for 22 to 25 minutes, or until the tops are set and lightly golden.
09 - Allow the baked oatmeal cups to cool in the tin for 5 minutes, then transfer to a wire rack. If desired, top each with chopped pecans or walnuts and drizzle with maple syrup before serving.