Preheat the oven to 425°F. Line a standard 12-cup muffin tin with liners and set aside.

In a small bowl, whisk together the flour, sugar, salt, and cinnamon. Add the butter and use your fingertips to rub the mixture together until pea-size crumbs form throughout and the mixture is no longer sandy. Set aside.

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

In a large bowl with a hand mixer (or in a stand mixer), cream together both sugars and butter on high speed for 2 minutes. Add the eggs one at a time, beating to incorporate between each addition. Beat in the vanilla extract. Add sour cream and milk and beat on medium speed until well blended.

Add dry ingredients to the butter mixture in 3 additions, gently mixing on low-medium speed. DO NOT OVER MIX- stop when it comes together and there are no dry streaks- the batter will be thick. Scrape down the bowl.

Use a spatula to gently fold the diced apples into the batter until just combined. Divide the batter evenly between the lined muffin cups. Top each muffin with about 1 tablespoon of the crumb topping, keeping it toward the center. Lightly press the crumbs down.

Bake at 425°F for 15-18 minutes or until the tops are golden and a toothpick inserted into the center comes out clean. Let the muffins cool in the muffin tin for 10 minutes, then transfer to a wire rack and enjoy.