Purple Sweet Potato Layer Cake (紫薯千层蛋糕)

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This cake is made for my Dad’s birthday, he likes old school desserts with pandan/ yam/ sweet potatoes/ coconut etc. Me, on the other hand, has been wanting to use purple sweet potatoes in my recipes because they yield such a beautiful intense colour. I thought this cake really satisfied both our requirements.

For the reader’s reference, I got my purple sweet potatoes from NTUC finest and they are originally from Vietnam. In this recipe, only less than 2 sweet potatoes were used for both the cake layer and the sweet potato layer, and yet the colour is really unbeatable. The taste is custardy and soft, and feels a bit like eating bo-bo-cha-cha jelly.

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The recipe is very similar to the very popular pandan kaya layer cake. Just note that when the sweet potato paste thickens at the last stage of making, work fast as it becomes less flowy by the minute. Focus on getting the sweet potato paste to flow to the sides of the cake. You can also see my mistake of having the cake layer exposed at the side, this can be solved by trimming the cake a little smaller.

Recipe (For an 8″ cake, adapted from Alex Goh’s Yam Layer Cake)

Cake

  • One 8″sponge cake (from here or your own)

Purple Sweet Potato Layer

(A)

  • 600g water
  • 125g sugar
  • 1/2 tsp salt
  • 1.5 tsp agar-agar powder

(B)

  • 270g coconut milk
  • 220g purple sweet potato puree (see more instructions below)

(C)

  • 100g fresh milk
  • 60g corn flour
  • 2 tsp pure coconut oil (optional, for fragrance)

Method

1. Prepare cake in advance. Using the recipe from Sweet Purple Potato Coconut Chiffon, bake it in a 8″round pan for 40 min at 170 deg C. Test with skewer.

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2. Cool completely and slice into 3 even layers. Test that they have shrunk slightly and fits into a 8″ pan. If not, also trim the sides slightly to ensure they fit.

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3. Next, for the sweet potato (instructions also available in the cake recipe link, so you can do this step together), boil with skin intact till soft. Peel, mash and strain through a wire sieve. Measure out 220g. There is no need to leave to cool, you can use this immediately.

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4. Mix all the ingredients in (A) in a big pot, (B) ingredients in a bowl and (C) ingredients in another separate bowl first. Prepare a clean 8″round cake pan. Rinse and make sure the pan is wet with water droplets (important to help unmoulding).

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5. Heat (A) over stove and bring to boil then turn to low heat. Do ensure all agar agar powder is dissolved to ensure that the potato layer solidifies properly.

6. Mix (B) ingredients to ensure they are combined. Add (B) to (A) and bring to boil again and turn to low heat.

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(B) ingredients

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7. Ensure (C) ingredients are well mixed as the corn flour tends to settle to the bottom after leaving to stand for a few minutes.

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(C) ingredients

8.  Ensure the wet cake pan and cake layers are all prepared and placed close by. Add (C) ingredients in a small steady stream to the pot on low heat. Stir for a 1-2 minutes on very low heat will do. The paste will thicken very fast. At first, the mixture will look very lumpy, stir it a bit more and as it cools later, the lumps will disappear.

9. Using a ladle, place about 1/3 of sweet potato mixture into the cake pan, follow this by a slice of cake. Press gently on the cake for 30 sec – 1 minute to ensure cake does not ‘float’ up as the sweet potato mixture solidifies.

10. Follow with another thin layer of sweet potato mixture. Ensure the mixture gets to gap between the cake and the side of the cake pan. The mixture is not very flowy so use the ladle or another spoon to help. If necessary, push the cake at the edge aside slightly to ensure the sweet potato mixture flows through.

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11. Repeat with another cake layer and press with fingers gently for one minute to firm up its position so it will not move around later.

12. Repeat with another thin layer of potato mixture (again, get it into all the gaps) and then the last slice of cake layer. You should have some potato mixture left (about 100-200g). Spoon some to the edges if necessary to fill up any empty space (see below, with lots of gap to fill). Work quickly. There will be some potato mixture left over which you can fridge and eat like jelly.

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13. Fridge the cake in the chiller section overnight (at least 6 hours).

14. When ready to unmould, use a hair dryer to blow on all sides of cake tin. Use fingers to gently pull cake away from the sides of the tin to dislodge. Turn cake onto a cake board and it should slowly slide out. Serve chilled.

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