Claire Ptak of Violet Bakery in London says under-beat, under-fold and under-bake; plus chocolate, prune and rum cake with Rayna Jhaveri, Dan Pashman of The Sporkful on the science of snacks; Adam Gopnik of The New Yorker on cultural appropriation; and the best way to beat egg whites. Host Christopher Kimball and co-host Sara Moulton answer your cooking questions.