Cake rolls are holiday fun. Start with an easy sponge cake recipe and add the spices of the season. To make it ahead, freeze the cake (but not the bark) for 2 hours to set the frosting. Wrap it in plastic wrap; return to freezer for up to a week. To serve, thaw in the fridge overnight; let stand at room temp 30 minutes before adding chocolate bark and slicing.
Step 1: Whip
Fluffy egg whites are the key to making this cake light and airy. Add sugar slowly to avoid weighing down your whites.
Step 2: Roll
While the cake is still warm, roll it into a log. (The towel keeps the cake from sticking to itself.) Keep it rolled until cool. This trains the cake to hold its final shape.
Step 3: Unroll
Unroll your cooled cake.
Step 4: Spread
Spread the filling over cake to within 1/4 in. of the edges.
Step 5: Roll Again
Roll up the cake again; trim ends and transfer to a platter.
Step 6: Frost
Spread the chocolate buttercream frosting on the cake.
Step 7: Spread Chocolate
For optional bark: Use an offset spatula to thinly spread 4 to 6 oz. melted bittersweet chocolate on a parchment paper-lined baking pan. The thinner the chocolate, the easier it is to break into shards. Refrigerate until set.
Step 8: Snap
This is the fun part. Get cracking. Uneven pieces add lots of barklike character. If chocolate softens, pop it back in the fridge.