Preheat oven to 350oF (180oC).
Grease sides and bottom of round cake pan (7 inch, 18 cm diameter). Line bottom of pan with parchment paper. Set aside.
To a bowl, add sugar and water. Mix until sugar dissolves completely.
Then, add oil and vanilla extract. Mix well.
Sift in flour and baking powder.
Mix until dry flour is no longer visible. DO NOT OVER MIX (mix until dry and wet ingredients are just combined).
Add sliced apples and gently fold into batter.
Transfer batter to prepared cake pan. Spread batter evenly.
Top cake with sliced apple.
Bake in preheated oven at 350oF (180oC) for 45 minutes.Then sprinkle top of cake with granulated white sugar.Bake at 350oF (180oC) for an additional 5 minutes or until toothpick inserted in the middle of cake comes out clean. Let cake cool down for 5-10 minutes in pan. Then, transfer to wire rack to cool down completely.