Toss the melted butter, flour, brown sugar, cinnamon, and salt together with a fork until crumbly.
To make the sour cream coffee cake muffins:
Preheat the oven to 425 degrees F and line a muffin pan with papers.
Place the flour, sugar, baking powder, and salt in a large bowl and whisk to combine.
In a smaller bowl, whisk the melted butter, sour cream, milk, eggs, and vanilla together until smooth.
Add the sour cream mixture to the dry ingredients, and fold together until just barely combined.
Fill the wells of the muffin pan halfway with the sour cream coffee cake batter, then top with a few teaspoons of the cinnamon crumb topping.
Divide the remaining batter equally among all 12 wells of the muffin pan, and top with the remaining cinnamon crumb topping.
Bake the crumb cake muffins for 5 minutes at 425 degrees F, then turn the oven temperature down to 350 degrees F and continue to bake for 14 to 18 minutes, or until a toothpick inserted in the thickest part of a muffin comes out clean or with a few moist crumbs.