Preheat the oven to 350°F and grease a large ramekin (see note) with butter or cooking spray. You can also add a circle of parchment paper to the bottom for even easier removal.
In a medium bowl, mix together the softened butter and oil with a fork until combined then mix in the sugar until pale and slightly fluffy. I like to mix it against the side of the bowl with the fork.
Mix in the egg white, milk, and vanilla extract until combined.
Fold in the flour, baking powder, and salt with a rubber spatula until combined. Do not over mix.
Pour the cake batter into the greased ramekin.
Bake for 22-27 minutes (I did 24 minutes, but time will vary based on ramekin size and oven) until it is golden on top, a toothpick comes out with just moist crumbs, and if you gently press the top of the cake it slightly bounces back.
While the cake cools, make the frosting. Stir together the butter and milk with a fork in a small bowl - it doesn’t need to be completely combined. Mix in the powdered sugar and vanilla. You can add more or less powdered sugar/milk to adjust the consistency.
Either pipe on the frosting or spoon it on top of the cooled cake and enjoy!
