Heat the oven to 350 F (180 C / Gas 4).

Grease and flour an 8 or 9-inch square baking pan.

In a mixing bowl with electric mixer, cream butter and sugar until light and fluffy, about 4 to 5 minutes.

Beat the egg into the creamed mixture until blended.

Measure the milk into a measuring cup. Stir the 1 teaspoon of vanilla into the milk.

In another bowl, combine the flour and baking powder.

Beating slowly, add about one-third of the flour mixture to the creamed mixture, along with half of the milk. Repeat with another one-third of the flour mixture and the remaining milk. Add the remaining flour mixture and beat until smooth and well blended

Pour the batter into the prepared baking pan.