Blend half a package (approximately 2 1/4 c.) cake mix, water and egg in mixing bowl.
Beat at medium speed until batter is smooth.
Pour half the batter into a greased 9" square pan.
Combine remaining cake mix with cinnamon; add melted butter and vanilla.
Toss with a fork to form large crumbs.
Sprinkle half of crumb mixture over batter in pan.
Arrange 1 c.of apple slices over crumb mixture.
Carefully spoon on remaning batter.
Arrange rest of apple slices over batter.
Dot with the butter and sprinkle with remaining crumb mixture.
Bake in 350° oven 40-45 minutes or until done.