Chocolate Sandwich Cake with Chestnut & Whisky Cream Filling

So I was making a birthday cake. I knew I wanted it to be a chocolate cake, but the person who normally bakes the cakes in this house was the person whose birthday it was and I was nervous in the run-up to the event, never having baked a cake before. I had my eye on a recipe in Jacques Pepin's Complete Techniques for a fridge cake involving a lot of chocolate, some butter, some rum and that tin of Clement Faugier Sweetened Chestnut Purée that had been sitting at the back of the food cupboard for about a year without my having any idea what to do with it. Actually, what I really wanted to cook was Anthony Bourdain's Charlotte de marrons, but I couldn't find sponge fingers anywhere.

In the end, I decided that the fridge cake would be excessively alcoholic, so I decided to cook a cake I had at least seen being baked before, and then modify it somewhat. So, this is a modified version of Nigella Lawson's chocolate Birthday Cake from How To Eat; the filling is my own, inspired my Messrs Pepin and Bourdain, and is based on my wish to use both chestnuts and alcohol to make the cake a bit more a.) seasonal and b.) interesting.

For the cake:

225g self-raising flour
30g best cocoa
200g caster sugar
100g unsalted butter
200g condensed milk
100g best quality dark chocolate
2 eggs, beaten

For the chocolate ganache:

100g best quality dark chocolate
100g best quality milk chocolate
200ml double cream

For the filling:

1 tin of sweetened chestnut purée (i.e. the stuff I mentioned above, or something similar - don't use use tinned chestnut purée that they sell for Christmas stuffing. The stuff you want has been heavily sweetened and flavoured with vanilla and other things and is a thick, glossy, sticky sludge when you open the tin.)
200ml double cream
1 double measure of whisky/rum/brandy

[The following method is mostly Nigella, with my annotations.]

Preheat the oven to 180C/Gas mark 4. Put the kettle on. Butter two shallow sandwich tins. [Nigella says line the base with baking parchment. I didn't, but I did use a carving knife to help get the cakes out of the tins when I'd finished.]

Sieve the flour, cocoa and a pinch of salt together into a large bowl and set aside.

Put the sugar, butter, condensed milk, 100ml just-boiled water and the chocolate, broken into small pieces, into a saucepan and heat until melted and smooth. Then stir this into the flour-cocoa mixture with a wooden spoon, and when all is 'glossily amalgamated', beat in the eggs.

Pour into the sandwich tins and bake for 20 minutes, no longer.

Leave to cool in the tins and then turn out onto a rack.

To make the ganache, break up the chocolate into small pieces (you could blitz it in a blender if you like) and put it in a medium-sized bowl. Heat the cream to boiling, but don't let it boil [that doesn't really make any sense, Nigella] and then pour it over the chocolate. Leave for five minutes and then, preferably with an electric mixer, beat until combined, coolish, thickish and glossy.

To make the filling...well, I sort of winged it, but here's how. The purée will come in a 500g tin and you need about 300g. Scoop it out into a medium-sized bowl. Add the liquor of choice and mix it in with a fork so that it's absorbed. Whip the cream with your electric mixer (you've washed the mixer bits since making the ganache, obviously) and then gently amalgamate the whipped cream into the chestnut/liquor goo until you get a thick but spreadable, pale brown cream. Yes, you could leave out the alcohol but don't do so on the grounds that it's more healthy: there is absolutely nothing healthy about this cake, and anyway the chestnut filling will seem tooth-hurtingly sweet without it.

It's important to wait until the cakes are cool before you put them together, otherwise the filling might melt. Spread the top half of one of the cakes with the chestnut/whisky cream. Be generous. Pretend it's the biggest and richest peanut butter sandwich ever. Gently lower the other cake on top and press down very gently (i.e. don't crush the damn thing). Lastly, spread your ganache on top of the upper surface of the upper cake (and the sides too, if you can be bothered) and let it set. It's probably a good idea to store this cake in the fridge so that the cream doesn't go off, but on the other hand that's liable to make the cake go stale quicker, so you should really try to eat it as soon as possible. I hope that's not too much trouble.