Preheat the oven to 350°F and grease a large ramekin with butter or cooking spray.
In a medium bowl, mix together the oil, egg white, vanilla, and milk with a fork to combine.
Mix in the sugar then fold in the flour, cocoa powder, baking powder, and salt until just combined.
Pour the batter into the greased ramekin.
Bake for 22-26 minutes until the cake is set on top, a toothpick comes out with just moist crumbs.
While the cake cools, make the frosting by stirring together the butter and milk in a small bowl.
Mix in the powdered sugar and cocoa powder.
Pipe or spoon the frosting on top of the cooled cake and enjoy!
