Melt the butter in a small bowl. Add the remaining ingredients and stir until well combined. Set aside.
Make and bake the muffins:
Preheat oven to 350 degrees.
Cream together the butter and granulated sugar until smooth.
Add the egg yolk and beat until pale yellow.
Add the remaining ingredients and stir until well combined, being careful not to overmix.
Divide the cake batter evenly among five greased muffin cups. Sprinkle crumb topping over the batter.
Bake the muffins 25-30 minutes, rotating pan halfway through. The muffins are done when a wooden skewer or toothpick inserted into the center comes out clean.
Let muffins cool slightly before removing them from the pan.
Notes
*I doubled the amount of butter for a richer, moister cake. If you're not watching your waistline, go ahead and use the whole stick. **I rarely buy buttermilk because it’s so easy to make with ingredients you likely have on hand. Just pour 1 tablespoon lemon juice or white vinegar in a measuring cup and add enough milk to hit the 1-cup line. Let the mixture sit at room temperature for 5-10 minutes until curdled.
Recipe by Two of a Kind at http://www.twoofakindcooks.com/cinnamon-crumb-coffee-cake-muffins/