Apple-Cinnamon Muffins

Preheat the oven to 375°F (190°C). In a large bowl, whisk together the flour, baking powder, cinnamon, and salt. In a separate bowl, beat the eggs and granulated sugar until well combined. Melt the butter and add it to the egg mixture along with the milk and vanilla extract. Mix well. Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Gently fold in the chopped apples. In a small bowl, mix together the brown sugar and ground cinnamon. Spoon the muffin batter into a greased or lined muffin tin, filling each cup about 2/3 full. Sprinkle the brown sugar and cinnamon mixture over the top of each muffin. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Allow the muffins to cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely.