A bit Googling and I found out 1 US cup = 235ml. Therefore, I mixed a another batch of 35g custard powder with a bit a milk. Mix them into a smooth paste and add them to my warm pot of “custard sauce” mixture. After a bit of constant stir under gentle heat, it turns into a thick custard.
https://www.amazon.com/Birds-Custard-Powder-Ounce-Canisters/dp/B00HW87CB8

