Preheat your oven to 350°F (175°C) and line a cupcake pan with paper liners.

In a mixing bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.

In a separate bowl, cream the softened butter and granulated sugar until light and fluffy (about 3-4 minutes).

Beat in the eggs one at a time, adding vanilla extract after the second egg. Mix until fully incorporated.

Gradually alternate adding the dry flour mixture and buttermilk into the wet ingredients, starting and ending with the flour mixture. Mix until just combined; be careful not to overmix.

In another bowl, combine diced apples with lemon juice, brown sugar, and cinnamon. Toss gently to coat.

Fill each cupcake liner about halfway with the cupcake batter, then spoon a generous amount of the apple mixture on top, followed by an additional dollop of batter to cover the apples.

In a small bowl, mix together rolled oats, flour, brown sugar, cinnamon, and melted butter until crumbly. Sprinkle this mixture generously over each cupcake.

Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean. The crumble should be golden brown, and the cupcakes may spring back when gently pressed.

Once baked, allow the cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.