Preheat oven to 425°F. Spray a 12-count muffin pan with nonstick spray or line with cupcake liners. Set aside.
In a medium bowl, combine the flour, baking powder, baking soda, cinnamon and salt.
In bowl of mixer, cream together the butter and sugar until combined, about 2-3 minutes. Add in the eggs, sour cream, milk and vanilla extract and beat on medium speed until the texture is uniform. Add in the dry ingredients, folding with a spatula until just combined.
Spoon batter into prepared muffin pan, filling them all the way to the top.
Make the streusel topping by mixing together the flour, sugar and cinnamon in a medium bowl. Add the melted butter and mix until combined. Sprinkle each muffin generously with the streusel topping.
Bake for 5 minutes at 425°F and then reduce heat to 350°F. Continue to bake for 12-15 minutes or until a toothpick inserted in the center comes out clean. Allow muffins to cool for 5 minutes in pan before transferring to a wire rack to cool.
Notes
All refrigerated items should be at room temperature, which helps the batter mix easily and evenly.
Use an ice cream scoop to get an even amount of batter in each muffin tin.
You'll know the muffins are done when a toothpick comes out clean.