Add the ingredients for the cinnamon swirl to a small bowl and stir to completely combined. Add the ingredients for the cinnamon sugar to another small bowl and stir to completely combined. Set both bowls aside.
Preheat the oven to 350℉ and fill 12 muffin cups with paper liners or line a sheet pan with silicone muffins cups.
Make the Muffins
In a medium bowl, thoroughly combine the flour, baking powder, cinnamon, and salt.
In a large bowl, whip the butter with sugars. Add eggs, milk, and vanilla and beat until smooth.
Stir the dry ingredients into the wet ingredients until just combined. Fold in the apple and the brown sugar swirl.
Spoon the batter into the muffin cups and sprinkle with the cinnamon sugar. Bake the muffins in the center of the oven for about 20-22 minutes, until a toothpick inserted in the center comes out clean. Let cool slightly before serving. Store in an airtight container.