Preheat oven to 350ºF.
Using an electric mixer, cream butter and sugar until light and fluffy. Beat in egg until fully incorporated.
In another bowl, combine flour and baking powder. While beating slowly, add about 1/3 of the dry mixture to the creamed mixture and half of the milk. Repeat this step, with another 1/3 of the dry mixture and the remaining milk; then add the remaining dry mixture and beat thoroughly until fully incorporated.
Pour batter into a floured and greased square 9-inch baking pan. Bake for about 30 minutes.