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 milk.
Fold in the flour, baking powder, and salt and mix until combined and smooth.
Mix together the cinnamon sugar in a small bowl.
Pour about half of batter into the greased ramekin and spread into an even layer. Sprinkle with about half of the cinnamon sugar. Top with the other half of the batter and sprinkle on the remaining sugar. Use a butter knife to swirl through the batter.
Bake for 22-26 minutes until a toothpick comes out with just moist crumbs.
Enjoy!
