Toasted Marshmallow and Rhubarb Cake

Make Ahead – Bake sponges up to 1 month ahead. Once cold, wrap each sponge in double layer of food wrap. Freeze on baking sheet until solid, remove from baking sheet, wrap in layer of foil and freeze until needed. Freeze egg whites in grease-free, airtight container for up to 1 month. Defrost egg whites overnight in fridge and bring to room temperature before making frosting. To assemble cake, make frosting as directed in recipe. Remove sponges from freezer and assemble cake while the sponges are still frozen and frosting is slightly warm. Toast frosting then let cake stand for 1–2 hours before slicing. The cake with frosting is best on day it is made.