• Preheat oven to 425°F and line a muffin pan with paper liners; spray liners lightly with nonstick cooking spray. Melt butter and allow to cool.

Make streusel: 

• Combine flour, brown sugar, cinnamon, and salt in a small bowl; add 8 tablespoons melted butter and mix together with your fingers until mixture is crumbly. Chill in the freezer until needed.

Make muffins: 

• Whisk together flour, sugar, baking powder, baking soda, and salt in a large bowl. Combine eggs, sour cream, remaining melted and cooled butter, and vanilla in a medium bowl.
• Add dry ingredients to wet ingredients and fold in just until incorporated – the muffin batter will be thick. If you have time, let the batter rest for 15 minutes.
• Place 2 tablespoons of batter in the bottom of each muffin liner and smooth out with the back of a spoon. Top with 1 tablespoon of streusel topping.
• Add another 2 tablespoons of muffin batter and smooth the top. Finish each muffin with another tablespoon of streusel topping

Bake muffins: 

• Place muffin pan in the oven and bake for 5 minutes at 425°F. Without opening the oven door, reduce temperature to 350°F and continue baking for another 15 to 18 minutes, until muffins are golden, tops spring back to the touch, and a toothpick inserted in the center comes out clean or with a few crumbs attached.
• Let muffins cool in pan on a wire rack for 5 minutes before removing from pan to cool.