Preheat your oven to 350°F (175°C) and prepare a muffin tin with cupcake liners.

In a large mixing bowl, whisk together all-purpose flour, baking powder, baking soda, salt, and ground cinnamon.

In a separate bowl, cream the softened unsalted butter with granulated sugar until light and fluffy.

Add eggs one at a time to the butter-sugar mixture, mixing well after each addition. Then stir in vanilla extract.

Gradually add the dry ingredients to the wet mixture, alternating with buttermilk, beginning and ending with dry ingredients.

Fold in freshly grated apples, ensuring they're evenly distributed.

Spoon the thick batter into lined muffin cups, filling each about two-thirds full.

Bake in the preheated oven for 18-20 minutes; check doneness with a toothpick.

Cool the cupcakes in the pan for 5 minutes before transferring to a wire rack.

For the buttercream, beat softened butter until smooth, then gradually add powdered sugar, followed by cinnamon, milk, and vanilla extract.

Frost the cooled cupcakes generously with the cinnamon buttercream.