Prepare the filling: Melt the butter in a 13 x9x2-inch baking pan in the preheated oven. Add apples; toss to coat. Stir together the sugar and cinnamon and sprinkle over apples. Toss to mix.
Prepare the topping: Mix together the flour, sugar, baking powder and salt in medium bowl. add the butter, milk, eggs and vanilla. Beat the mixture for 2 minutes or until blended and smooth. Spread evenly over apples to cover.
Bake in oven for 40 minutes. Let stand for 15 minutes. Cut into squares. Spoon juices in the bottom of pan over cake. Refrigerate left overs.