Preheat the oven to 350°F and grease a large ramekin (see note for size) with butter. I also like to add a circle of parchment paper to the bottom for easier removal.

In a medium bowl, use a fork or rubber spatula to stir together the softened butter and sugar until creamed and combined (I like to ‘smoosh’ the mixture against the side of the bowl to really cream things together). Mix in the egg yolk and vanilla until combined then mix in the sour cream and milk. It may separate slightly and look 'curdled', this is normal.

Mix in the flour, baking powder, and salt until just 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. You can use a spatula to do this, but I find it easiest to do by hand. The texture should be like wet sand that can hold its shape. If it’s too dry, you can add a tiny bit of melted butter. Sprinkle the streusel on top of the coffee cake batter in both big and small clumps.

Bake for 24-28 minutes (I did 26 minutes, but will vary based on your ramekin size and oven) until a toothpick comes out with just moist crumbs (no wet batter). If you find the streusel is getting too dark, you can gently place a piece of foil on top while baking.

Allow to cool slightly before removing from ramekin and enjoying. It's easiest to remove from the ramekin while it is still warm.