Preheat the oven to 350°F and grease a large ramekin with butter.
In a medium bowl, use a fork or rubber spatula to stir together the softened butter and sugar until combined.
Mix in the egg yolk and vanilla until combined then mix in the sour cream and milk.
Mix in the flour, baking powder, and salt and mix until combined.
Pour the batter into the greased ramekin and spread into an even layer.
Make the streusel topping by mixing together the softened butter, brown sugar, cinnamon, and flour.
Sprinkle the streusel on top of the coffee cake batter in both big and small clumps.
Bake for 24-28 minutes until a toothpick comes out with just moist crumbs.
Allow to cool slightly before removing from ramekin and enjoying.
