If you want to propose to her on her birthday, have a birthday cake made for her with the words of proposal on it, have it delivered to her while you are with her, and then tell her that is your way of proposing to her.
There are 343 calories in 1 slice of King Cake. Get full nutrition facts and other common serving sizes of King Cake including 1 oz and 100 g. Register | Sign In
But generally, I would say, for a standard 9 cake, divided in 6, one of those slices will be about 500-600 vcals, but keep in mind, it is  considered a better for you cake, just don;'t eat too much!! Probably that little slice alone had roughly 150 calories in it, but enjoy it your birthday only comes once a year. I wouldn't really worry about the caloric intake in the cake...worry about the fat that you are consuming. There are lots of food out there that are low in calorie, but high in fat.