Pre-heat your oven (or toaster oven) to 350F. Spray cookware with non-stick baking spray, or use liners for cupcakes.
Combine flour, sugar, baking powder and salt in a bowl.
Combine beaten egg, milk and vanilla in a second bowl.
Pour the wet ingredients into the dry ingredients. Stir to combine. Do not over-mix.
Add melted butter to the batter, stirring to incorporate. If batter is very thick, add 1 additional Tbsp milk.
Pour batter into the prepared pan. Smooth the batter for a small sheet cake. Make a trough in the top of the batter for a loaf cake. Fill cupcake liners 2/3 full.
Bake until a tester inserted in the centre comes out clean. Cupcakes cook most quickly, a small sheet cake takes a little longer, and a loaf may take up to 30 minutes.
Cool on a rack for 5 minutes before turning out of the pan. Cool completely before frosting.