Preheat the oven to 375 degrees. Grease an 8-inch by 8-inch by 2-inch baking pan.
Make the batter: Sift the flour with the baking powder and salt; set aside. In a medium bowl, beat the egg until frothy; then beat in the sugar and butter until well combined. Add the milk and vanilla. With a wooden spoon, beat the flour mixture into the sugar mixture until well combined.
Pour into the prepared pan. Sprinkle with the topping mixture and bake 25 to 30 minutes or until a cake tester comes out clean. Cool partially in the pan, on a wire rack. Cut into squares while still warm and serve.