To make the cake, begin by greasing the cake pan with melted butter, and then line it with baking parchment.
Mix the currants, sultanas, raisins, mixed peel and glacé cherries together and pour the rum over top. Cover and set aside for a few hours or overnight. If there is a large amount of alcohol in the bottom of the bowl or pan when you go to make the cake, drain a bit of it off, but if there are only a couple of tablespoons you can safely leave it.
Mix the soaked dried fruit with the chopped and ground almonds. Set aside.
Sift the flour, cocoa, salt, baking powder and mixed spice together..
In a separate bowl, cream the butter and brown sugar.
Add the lemon rind.
Add the beaten eggs a little at a time, beating after each addition.
Fold in the flour mixture, the coffee essence and then the dried fruit and nuts.
Turn the mixture into the prepared cake pan and smooth over the surface, making a slight hollow in the centre so the cake will rise evenly.
Bake at 325ºF (160ºC or 150ºC for a fan oven) for 60 minutes, and then reduce the temperature to 300ºF (150ºC or 140ºC for a fan oven) for a further two hours, or until a skewer inserted in the centre of the cake comes out clean. Do keep an eye on the cake while it is baking as oven temperatures do vary radically and you don't want it to burn. I tend to check it every thirty minutes to forty minutes just to keep an eye on it, and every fifteen minutes towards the end.
Cool the cake in the pan for at least a half an hour and then carefully remove it to cool on a wire rack.
After the cake is completely cook you can wrap it in foil. If you are making it well ahead of time, you can keep it in a cool dry place for six to eight weeks, feeding it once a week with a bit more rum. Of course, if you make the cake closer to the time you want to serve it, you can still 'feed' it in this way once a week until you cover it with marzipan.
A day or so before serving, we traditionally cover our Christmas cakes with marzipan and royal icing. You can use store bought marzipan and royal icing if you prefer, but the traditional recipes follow.
To make the marzipan from scratch, mix the ground almonds, sugar and icing sugar together.
Rub or cut the butter into this mixture until it looks like coarse breadcrumbs.
Mix the egg yolks and lemon juice and add them to this mixture.
Work the paste to a stiff but pliable consistency, adding a bit more lemon juice if it seems dry.
Divide the almond paste in half.
Using a light dusting of corn flour (corn starch) to prevent sticking, roll the first half into a circle that will cover the top of the cake.
Brush the top and sides of the cake with the sieved apricot jam, and apply the circle to the top of the cake, pressing firmly.
Using your hands, roll the second half of the almond paste into a long roll and then flatten it as evenly as you can with a rolling pin. This piece needs to be wide enough to cover the sides of the cake.
Press the flattened roll along the sides of the cake, pressing firmly and sealing the joins between the top and sides with your fingers.
Leave the cake in a cool airy place for several hours so the almond paste can dry out a bit.
To make the royal icing, beat the egg whites lightly.
Add the lemon juice and glycerine.
Beat in enough icing sugar to make a thick coating consistency that rolls easily off a wooden spoon, but leaves it coated.
Place the marzipan topped cake on a piece of greaseproof or waxed paper. Have a palette knife to hand to help guide the icing. Pour the icing bit by bit over the top of the cake, spreading it with the knife. Let it find its way down the sides, guiding it with the knife until the cake is completely covered. Leave the cake in a cool airy place overnight, or until the icing has set.