Thank you all! 
I am thinking maybe my cooling rack is very near to the counter and maybe this is the way that causes the moist surface when it is closer to the counter surface when overturn. After reading all your kind inouts, I didn’t know that sugar plays such a role. So…. the other cakes I make also has a sticky surface but not so moist like the cake I make yesterday, is all because of the sugar. *enlighten*
Kathleen - How fast you usually refrigerate the cake when it is cool? Does it need to be out from the cake tin? So can I fridge the cake till I am ready to frost the cake right?
The recipe I am using,
Ingredient A:
120g superfine flour
2g baking powder
50g castor sugar
1g salt
Ingredient B:
80g melted butter
75g egg yolk
60g milk
Ingredient C:
150g egg white
70g castor sugar
1g cream of tartar
1. Combine ingredient A.
2.Add in ingredient B and blend to form soft batter.
3. Beat ingredient C until stiff peaks form.
4. Gradually fold in 1/3 of egg white form into the batter, stir gently, repeat the process, mix well after each addition.
5. Pour the batter into a 9” lined cake tin.
6. Bake at 180degC for 30-35min.