For the Muffins

1. Prepare your oven. Turn it on to 400°F (200°C) to heat up. Line a standard muffin pan with paper liners or spray the cups well with non-stick cooking spray.
2. In a large mixing bowl, measure out the flour, sugar, baking powder, baking soda, and cinnamon. Use a whisk or spoon to combine these dry ingredients thoroughly.
3. Add the sour cream, melted butter, and eggs to the bowl with the dry mixture. Stir everything together until the ingredients are just combined. Be careful not to mix the batter too much.
4. Gently fold in the diced pieces of apple using a spatula. Distribute the apple evenly through the batter.

For the Muffin Topping

1. Get ready to make the crumb topping. Take the cold butter and cut it into tiny pieces. Place the cut butter into a small bowl with the sugar, flour, and cinnamon meant for the topping. Use a fork or your fingertips to blend the ingredients until the mixture looks like coarse crumbs.
2. Use a spoon or an ice cream scoop to fill each prepared muffin cup with batter. Fill them about two-thirds of the way full.
3. Take the crumb topping mixture and sprinkle a generous amount over the top of the batter in each muffin cup.
4. Carefully place the filled muffin pan into the preheated oven. Bake for about 24 to 28 minutes. You can tell the muffins are ready when a small wooden skewer or toothpick inserted into the center of one comes out clean with no wet batter attached.