1 large apple, cored, peeled and cut into 1/2-inch pieces
Prepare the Brown Sugar Swirl
Preheat the oven to 350° and fill 12 muffin cups with paper liners. In a small bowl, combine the brown sugar with the flour, a pinch of salt and the cinnamon.
Prepare the Cinnamon Sugar
In another small bowl, whisk the granulated sugar with the cinnamon.
Make the Muffins
In a medium bowl, whisk the eggs with the butter, milk and vanilla until smooth.
In a large bowl, whisk the flour with the granulated sugar, brown sugar, baking powder, cinnamon and salt. Using a wooden spoon, stir in the wet ingredients until 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 22 minutes, until a toothpick inserted in the center comes out clean. Let cool slightly before serving.