01 - Set oven to 375°F and line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - Whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon in a large bowl.
03 - In a separate bowl, whisk eggs, vegetable oil, milk, and vanilla extract until blended.
04 - Pour the wet mixture into the dry ingredients, stirring gently until just combined without overmixing.
05 - Gently fold diced apples into the batter ensuring even distribution.
06 - Spoon batter evenly into prepared muffin cups.
07 - Mix brown sugar and cinnamon in a small bowl and sprinkle evenly on muffin batter tops.
08 - Bake for 18 to 20 minutes until a toothpick inserted in the center comes out clean.
09 - Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.