Mix the egg, milk, butter, and vanilla together in a small bowl.
Combine the flour, sugar, baking powder, and salt in a mixing bowl and add the egg mixture.
Mix with just a few strokes until combined.
Pour into a greased 9-inch (23 cm) cake pan at least 2 inches (5 cm) deep.
Arrange the apple slices on top of the batter and sprinkle with the cinnamon sugar.
Bake in a preheated 375F (190C) oven for 25 to 30 minutes, until a toothpick inserted in the center comes out clean.
Serve warm or at room temperature.
Makes on 9-inch (23 cm) cake to serve 6 to 8.