HAVING YOUR CAKE: If you’re a mother or little girl in possession of a terrific recipe for chocolate cake, Alber Elbaz and Lanvin want to know. To trumpet the arrival in boutiques of its new children’s wear range, Lanvin Petite, the French fashion house is conducting an online recipe contest for mothers and daughters. The winner is to be chosen by the executive chef at the Ritz Escoffier cooking school, which will then whip up the cakes to serve at a luxury tea party at the famous Place Vendôme hotel on Dec. 4. The children’s wear is already available at Lanvin’s online shop, and is to land in its flagship Paris boutique on the Faubourg Saint-Honoré next week. Deliveries to select boutiques worldwide are to follow in mid-December.