Toasted Marshmallow and Summer Berries Cake

You do need to be prepared to whisk the whites until truly thick, but if you have a mixer, this does most of the work. The yolks are used to make two tender and celestially light golden sponges, and the rhubarb that goes between them provides its emphatic tang, offsetting the intense sweetness of the marshmallow icing. Out of forced-rhubarb season, I favour a mixture of