Coat a 9-inch round cake pan with cooking spray. Place 1 1/2 tablespoons melted butter in pan. Quickly tilt pan in all directions so butter covers pan with a thin film. Sprinkle butter with 2 tablespoons brown sugar. Arrange apple slices spoke-like over brown sugar. Set aside.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 3 ingredients in a medium bowl.
Beat softened butter, apple butter, and 2/3 cup brown sugar in a large bowl with a mixer at medium speed until well blended. Add egg, vanilla, and buttermilk; beat well. Gradually stir in flour mixture. Spread batter over apples.
Bake at 350° for 23 to 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 5 minutes. Loosen cake from sides of pan using a narrow metal spatula. Place a plate upside down on top of cake pan; invert cake onto plate. Serve warm.