Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch pan.
In a medium bowl, stir together the flour, baking powder and salt. Set aside.
In a large bowl, cream together the butter and sugar until smooth. Beat in the egg and vanilla. Gradually blend in the dry ingredients. Spread half of the mixture into the prepared pan.
Bake for 15 minutes in the preheated oven. Sprinkle with apples, then top with remaining batter. Bake for an additional 25 minutes.