For the Cinnamon Streusel

1. In a medium bowl, add all purpose flour, granulated sugar, light brown sugar, cinnamon, salt, corn starch, and slightly cooled melted butter. Use a fork to rake the butter through the ingredients. Continue mixing until the mixture combines into formed crumbs and there are few pieces of the dry flour mixture.

For the Muffins

1. Preheat your oven to 375 degrees Fahrenheit and line a 12 count muffin tin with liners.
2. In a large bowl, whisk melted butter, granulated sugar, room temperature sour cream OR greek yogurt, room temperature milk, room temperature eggs, and vanilla extract together until smooth.
3. Add all purpose flour, baking powder, baking soda, and salt to the bowl and whisk together only until you see the ingredients disappear to avoid overmixing. Use a spatula to scrape down the sides and the bottom of the bowl to ensure the ingredients are properly mixed throughout.
4. Divide the batter between the 12 liners, adding about 1/4-1/3 cup of batter to each liner. The liners should be almost full.
5. Top the muffins with about 1 1/2 tablespoons of the crumble topping.
6. Bake the muffins for 15-18 minutes or until the tops are light golden brown, domed, and the centers spring back when poked with your pointer finger. A toothpick inserted into the center should come out with some crumbs.
7. Store the muffins tightly covered, in an airtight container, or individually wrapped with cling wrap at room temperature for up to 3 days or in the freezer for up to 4 months. Thaw at room temperature or in the microwave.