Combine the sugar and cake flour in a bowl, and lightly mix together with a whisk. Break up any lumps of sugar or flour.
Add the milk and egg, mixing after each addition. Strain the mixture through a sieve, and put it in a heat resistant bowl.
Without covering with cling film, microwave the mixture for about 2 minutes at 600 W. When the center of the bowl has started to rise, take it out from the microwave, and mix with the whisk until the mixture is smooth.
Put the mixture back into the microwave, and heat for 1 minute. Mix with the whisk once more until it's smooth.
Add the coconut milk. Coconut milk is easier to mix into heated ingredients, so I added it here.
Put the mixture back into the microwave. Microwave for 30 seconds to 1 minute. When the center of the mixture starts to form lumps, or when the mixture starts to bubble, take it out from the microwave and mix. (Be careful not to burn yourself!)
Repeat Step 6 until the mixture reaches your desired consistency. When it's cooled a little, add the umeshu for some extra flavor and chill in the fridge.
[Yogurt Cream] Drain the yogurt for half a day to overnight. Add the drained yogurt to the basic cream, followed by the sugar, and mix together.
I made this roll cake with a sponge cake and canned mango. This cream goes really well with tropical fruit.
Story Behind this Recipe
I tried making Kaya jam at home, but I failed when I made it in a pot since the mixture didn't come together. The ingredients and cooking steps are similar between Kaya jam and custard cream, so I thought it might work if I made basic custard cream first, then add coconut milk using a microwave.
I used a large egg. I wrote down the amount of coconut milk as 50-100 ml, but the more you use, the stronger the coconut flavor becomes so adjust the amount to your liking. Drain the yogurt really well. If the yogurt isn't drained properly, the cream will be runny and watery.