To decorate the tart, trim the edges of the cooled tart with a serrated knife, cutting away from you so any bits of pastry don’t fall into the tart, then lift off the ring. Take your block of chocolate and scrape off curls using a knife or a potato peeler. Hold the flat side of the knife against the chocolate and pull the blade along the surface either towards you or in an arc. Tip off the curls and repeat. You’ll find the chocolate responds best when it is at room temperature – if your curls are shattering, move the chocolate somewhere a little warmer and wait a few minutes before trying again. Arrange the curls on the tart and for a christmassy effect, dust the lot with icing sugar. To divide the tart evenly, cut it in half and then cut each half into 4 or more. And my final tip, cut the tart with a hot knife so it slides through the chocolate smoothly.