Whisk together the egg and sugar; add the vanilla and vegetable oil. Add the flour, baking powder and salt; whisk until just combined. Add the milk and whisk again.
If making the ball cake, spray three cavities of a 3 1/2-inch ball pan with flour-based baking spray (or grease and flour them). Pour batter into the cavities 3/4 full. If making cupcakes, fill paper liners 2/3 full.
Bake the cakes for 15 to 20 minutes, or until well risen. When done, the cakes should spring back when pressed in their centers.