The easiest and loveliest chocolate cake is, I think, Nigella's recipe from How to Eat:
Birthday Cake
from How to Eat by Nigella Lawson
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
250g best quality chocolate
250ml double cream
Preheat the oven to 180 degrees Celcius. Put the kettle on. Butter a 20cm springform cake tin (or 2 sandwich tins) and line the base with baking parchment.
Sieve the flour, cocoa and a pinch of salt together in a large bowl and set aside.
Put the sugar, cocoa, butter, condensed milk, 100ml just boiled water and the chocolate broken into small pieces in a saucepan and heat until melted and smooth. Then, using a wooden spoon, stir this robustly but not excitably into the flour-cocoa mixture and, when all is glossily amalgamated, beat in the eggs.
Pour into the cake tin and bake for 35-45 minutes; less if you're using the shallower sandwich tins. When it's ready, the top will feel firm.
Leave to cool in the tin for 10 minutes and then turn out onto the rack.
When completely cool, split in half horizontally or, if using the sandwich tins, stick the two cakes together.
To make the ganache, chop up the chocolate and put it in a medium sized bowl, preferably a wide shallow one rather than a pudding basin shape. Heat the cream to boiling (but do not let it boil) and pour it over the chocolate. Leave for 5 minutes and then, using an electric mixer, beat until combined, coolish, thickish and glossy. You want it thin enough to pour but thick enough to stay put. At this stage, think on the ganache as somewhere between a sauce and an icing. Later, it will set hard. Pour some over the cut side of one half of the cake, using a palate knife to spread, and then plonk the other half on top. Pour the rest of the chocolate ganache over the top of the cake, letting it drap over.
Leave for a couple of hours or until set.