In a medium mixing bowl, beat together butter, sugar and egg with an electric mixer on medium speed until well combined.
Stir together flour, baking powder and salt; add alternately with milk to beaten mixture, beating on low speed after each addition just until smooth.
In a small saucepan, bring maple syrup just to a boil; pour into a greased 8" x 8" x 2" baking pan.
Carefully top with a single layer of apple slices. Pour batter evenly on top.
Bake in a 400 degree oven for 25-30 minutes or until a wooden toothpick inserted into cake portion comes out clean.
Immediately invert onto serving platter. Cool slightly.
Serve warm, topped with shipped cream or a scoop of vanilla ice cream.