Baked Potato Cake (土豆蛋糕)

This baked potato cake smells wonderful and it's very tasty.  Not difficult to do and it's unusal from normal cake.  Only important point to note is that need to make sure you test different areas of the cake to make sure it's fully baked before removing from oven.

(Receipt makes a 7 inches square tin cake)
Ingredients:
1 kg USA Potato (Peel, slice thick and steam 40 minutes.  Mash it while hot and leave aside)
250g Sugar
150g Wheat Flour (Deng Flour)
150g Soft Butter (Thaw in room temperature)
5 Egg Yolks (lightly beaten and add 1 tsp vanilla essence in it)
3 Egg Whites
* 1 Egg for brushing (lightly beat and sieve it)

• Preheat oven at 180 degree
• Beat sugar and butter together for 5 minutes with maximum speed.
• Add in egg whites slowly while still mixing and continue to beat 3 more minutes.
• Thereafter slowly add in egg yolk and continue to beat till well mixed.

• Add in mashed potatoes and mix well.
• Turn mixer to slow speed and then add in wheat flour slowly to mix well (don't beat too long for this process).
• Grease and line 7" suqare tin.
• Pour mixture to tin and bake for 10 minutes then remove and apply egg wash.  Continue egg wash process while baking every 10 minutes.   Bake for 30 minutes.

• Bake for another 15 minutes (or till cooked) after 3rd egg wash.  If skin is turning very golden before cake is fully cooked, cover the cake tin with aluminium foil and continue to bake till cooked.  (Prick with toothpick on different areas of cake to make sure it's fully cooked.)
• Transfer cake to wire rck to cool after baked.  Cut only after cool down completed.