Preheat the oven to 325 degrees F (165 degrees C). Grease a 9x13-inch baking dish.
2
Sift flour and baking powder into a small bowl.
3
Beat sugar, 1/4 cup margarine, and egg in a large bowl with an electric mixer until smooth. Add flour mixture in two batches, alternating with milk, stirring batter briefly after each addition just to combine. Pour into the prepared dish.
