Beat the puree in a bowl until it’s smooth, and then add the butter, beating again to make a well-blended mixture. Melt the chocolate and let it cool slightly, before adding it to the chestnut and butter in the bowl.
Beat in the rum, and spoon the chocolate mixture into a 23 x 10cm / 9 x 4 inches loaf tin, lined with clingfilm, in two batches, making sure the first layer reaches the corners and sides of the bottom of the tin before you smooth over the rest.
Wrap the overhanging clingfilm over the cake so that it is completely covered, and put it in the fridge to set for at least four hours, but a day or so in advance if you want.
Don’t take the loaf tin out of the fridge until you want to eat it, when you just unmould the cake, cut it into thin slices and serve with creme fraiche or sour cream.