Preheat your oven to 350°F (175°C) and prepare your muffin tin with grease or paper liners.

In a large mixing bowl, combine all-purpose flour, baking powder, and cinnamon. Whisk well to remove lumps.

In another bowl, whisk together granulated sugar, beaten egg, milk, and vanilla extract until combined.

Core, peel, and chop Granny Smith apples into small pieces and fold them into the wet mixture.

Pour the wet mixture into the bowl with dry ingredients and fold together gently until just combined.

Fill the muffin tin about two-thirds full with batter and add oats or chopped nuts on top if desired.

Bake for 18-20 minutes or until golden brown and a toothpick comes out clean.

Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.