Hokkaido Cupcake

Tried to make Hokkaido cupcake some time back in June. The cupcakes was not so popular then. Till recently there is a lot of hype on Hokkaido cupcakes that alot of bakeries ride on the hype. The cheapest I saw is $2 per cake. I've tried the one from Q Bread (which is the first I saw selling Hokkaido cupcakes) and it feels pretty normal, not as what I expected which was soft and fluffy. Polar cakes sells it too but they don't sell it separately. They sold in boxes of 6. It is a tad too much to finish.

Googled for the recipe to make the cupcake and I tried making it again for the 2nd time.

Recipe adapted from here.
Ingredients
Chiffon cake
3 egg yolks
20g sugar
35g corn oil
60g milk
70g cake flour
3 egg whites
25g sugar

Filling
60g dairy whipping cream
10g sugar
1tsp instant custard powder
Icing sugar, for dusting

Method
1. Pre-heat oven to 170C.
2. Arrange paper liners on baking tray.
3. Hand whisk egg yolk and sugar till pale in colour.
4. Add in corn oil and milk, mix well.
5. Sift in cake flour, stir to combine.
6. Beat egg white until foamy, gradually add sugar, and continue beat till soft peak form.
7. Take 1/3 of egg white and use a hand whisk to mix into egg yolk batter.
8. Fold in the balance egg white with a spatula till well combine.
9. Scoop batter into pre-arranged paper liners to about ¾ full.
10. Bake for 20-25 mins at middle rack.
11. Beat whipping cream with sugar till firm and stiff (over a bowl of iced water).
12. Add in custard powder, mix well.
13. Pipe custard cream into cupcake and dust with icing sugar. Refrigerated before consume.

Both times the cakes sank upon cooling. I googled to find out the reason behind it and improvised on the 2nd time but the results is still the same. Arrgh. I cann't seem to master baking nice cupcakes. My forte is still biscuits.