It is a pineapple bun with a piece of butter stuffed inside. They are known in Cantonese as boh loh yau (菠蘿油), in which boh loh means "pineapple", and yau (oil) refers to butter. - from Wikipedia..
The name "Yuan Yang", which refers to mandarin ducks, is a symbol of conjugal love in Chinese culture, as the birds usually appear in pairs and the male and female look very different. This same connotation of a "pair" of two unlike items is used to name this rice. Due to the meaning of love, this dish is often appeared in the wedding dinner.- from Wikipedia..