Start by chopping the apple into thin round slices and remove the core from each slice, use a fruit corer to help you. After that, set the apples aside.
Brush the unsalted butter or margarine all over a skillet, then sprinkle some sugar over the butter.
Place the apple slices on the skillet, one next to the other and set aside.
Now into a bowl, add the egg, the oil, the milk and the sugar, mix very well until combined.
After that, add the wheat flour, the baking powder and the vanilla, mix again until combined.
Next, pour all this batter into the skillet, over the apple slices.
Then take it to the heat, cover the skillet and bake the cake for 30 minutes
After this indicated time, pierce the cake and if the toothpick comes out clean, it's ready.
Serve.