This walnut cake keeps well at room temperature for up to 3 days, but if you're making way ahead, it's best to freeze. Wrap the cake securely in several layers of cling-wrap, and a final layer of tin foil and freeze for up to 3 months. To thaw, transfer to the chiller and let it thaw (still in its wrapping) overnight. Unwrap and let it sit at room temperature for at least 2 hours before decorating and serving.