我請你食件蛋榚 英文點講

I think it should be: "I treat you TO a piece of cake"
or simply "Would you like a piece of cake?", "Would you like some cakes?"

Other examples of the use of "treat"

I treat Bo Bo to dinner at Maxim's.
She treats people kindly
Many diseases can be treated with medicine

invite you to eat pieces of egg-cream

I think you can use "I treat you a piece of cake." or "I offer you a piece of cake."

好容易,do u need some cake?