To get a nice log shape, lay the cake on a kitchen towel, fill it, roll it, then clip the towel's ends together with clothespins until the cake sets. We used frozen currants as a garnish, but frozen grapes work just as well. You may substitute cream cheese for the mascarpone; add two more tablespoons sugar and a half teaspoon vanilla.
This Italian triple-cream cheese is technically a cheese but has the texture of a wonderfully thick cream. It adds delight to all manner of recipes: It's the essential element in tiramisu; makes a luxurious frosting for sugar-and-spice cake; is a super-easy tart filling; adds rich, creamy depth to pastry puffs; and is the essential ingredient in icebox cake, semifreddo, and more.
Mascarpone, a triple-cream cheese available at most grocery stores, adds a rich, silky component to a frosting. The texture is ideal for holding swoops and creating a snowdrift-like effect when slathered on with an offset spatula.