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

Make the crumb topping - In a small bowl, whisk together the 1/2 cup flour, 1/4 cup sugar, 1/4 tsp salt, and 1/2 tsp cinnamon. Add the butter and use your fingertips to rub the mixture together until pea-sized crumbs form throughout and the mixture is no longer sandy. Set aside.

Mix the dry ingredients - In a medium bowl, whisk together 2 cups flour, 1 1/2 tsp baking 
powder, 1/2 tsp baking soda, 1/2 tsp salt, and 2 tsp cinnamon. Set aside.

Mix the wet Ingredients - 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 1/2 cup sour cream and 1/4 cup milk and beat on medium speed until well blended.

Combine the batter - 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.

Add the apples - Use a spatula to gently fold the diced apples into the batter until just combined.
Divide the batter evenly between the lined muffin cups (a trigger-release scoop is helpful). It will seem like too much batter, but it works. 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.