When the cakes are cool (or, if wrapped in clingfilm, the sponges will mature to an even tastier cake in a day or two) remove from the tins and make the icing. Beat the butter with electric beaters until soft, then beat in half the icing sugar. Beat in the remaining icing sugar with the vanilla extract, until the icing is fluffy and pale. Beat in the 4 tbsp caramel. Spread a thick layer of icing over one of the cakes and chill for 20 minutes to firm up, then sandwich the second cake on top. Spread the rest of the icing sparsely over the top and sides of the cake, filling any gaps where the sponges were sandwiched and scraping excess icing to give straight, clean, ‘naked’ sides. Chill for 30 minutes until the icing is cold and firm.