Preheat your oven to 350°F (175°C). In a large mixing bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon. Set aside.

In another bowl, cream together butter and granulated sugar until fluffy. Add brown sugar, eggs, and vanilla extract, mixing well after each addition.

Gradually add the dry ingredients to the wet mixture, alternating with buttermilk, and mix until combined. Fold in grated apples.

Scoop batter into lined muffin cups, filling them 2/3 full. Bake for 18-20 minutes or until a toothpick comes out with a few moist crumbs.

Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

For the buttercream, beat softened butter until smooth. Gradually add powdered sugar, cinnamon, and milk until desired consistency is reached.

Frost cooled cupcakes with the cinnamon buttercream using a piping bag or spatula.