This article contains plot details from the latest book.
Custard bursts are pink and purple square puffs that look like marshmallows but are hard like candy on the outside with goo on the inside. They come in different flavors including chocolate, chocolate-cherry, butterscotch, lushberry, and caramel.
They used to be Jolie's favorite pastries. Edaline likes making them for Sophie, and they are probably Sophie's favorite dessert besides Mallowmelt. According to Shannon Messenger, they are based on french macarons.