Preheat oven to 375 degrees F. Butter the sides of a 9×13 inch baking dish.
In a bowl, cream butter and sugar together until light and fluffy. Beat in eggs, one at a time, then stir in vanilla. In another bowl, whisk
In another bowl, whisk together flour, baking powder and salt. Gradually add to butter mixture, mixing until just blended.
Spread two-thirds of the batter evenly in the prepared dish. Arrange apple slices over batter. In a small bowl, mix together brown sugar and cinnamon; sprinkle over apples. Spread remaining batter over apples.
Bake for 35 minutes, or until golden brown. Let cool for 10 minutes before serving."