Roasting rhubarb with a little sugar creates glazed pieces that retain their shape and look attractive in this layered dessert. I use this same custard when I make banana pudding, but when I pair it with rhubarb, I can't resist adding a few drops of rosewater to the recipe. I adore the subtle fragrance of rosewater in sweet and fruity recipes, as one often finds in Middle Eastern desserts; however, not everyone appreciates floral flavors in food, so it's fine to replace the rosewater with more vanilla extract instead. When you don't have time to fool with a trifle, just spoon the warm rhubarb over ice cream.