Below is what Nigel says about this cake ... I can't find fault with him, in any way. It was delightful.

However: I bought pre-toasted hazelnuts and accidentally put a mix of nutmeg and cinnamon into the cake (because I didn't read the spice jar label... it wasn't a disaster!)
I left the cinnamon out of the icing, and I dressed it with pomegranate. Yum!!

CHOCOLATE CHIP HAZELNUT CAKE WITH CHOCOLATE CINNAMON BUTTER CREAM by Nigel Slater

I like birthday cake. I love making them – and receiving them, too. This is my first choice for such an occasion. I serve it in appropriately thin slices.

ENOUGH FOR 10–12

Butter 250g
Golden caster sugar 250g
Shelled hazelnuts 75g
Dark chocolate 120g
Large eggs 4
Self-raising flour 125g
Ground cinnamon ½ tsp
Strong espresso 4 tsp

for the spiced chocolate butter cream
dark chocolate (70% cocoa solids) 250g
butter 125g
ground cinnamon a knifepoint

Set the oven at 180C/gas mark 4. Line the base of a 20-21cm loose-bottomed, deep cake tin with baking parchment. Cut the butter into chunks and put it with the sugar into the bowl of an electric mixer. Beat until white and fluffy. Toast the hazelnuts in a dry pan, then rub them in a tea towel until most of the skins flake off. Grind the nuts to a coarse powder. Chop the chocolate into what looks like coarse gravel.

Break the eggs into a small bowl and beat them gently. Slowly add them to the butter and sugar mixture, beating all the time – it may curdle slightly, but it doesn't matter. Stop the machine. Tip in half the ground nuts and half the flour, beat briefly and at a slow speed, stop the machine again, then add the rest, together with the chopped chocolate and cinnamon, and mix briefly. Gently fold in the espresso, taking care not to knock the air from the mixture. Scoop into the cake tin. Smooth the top and bake for 35 to 45 minutes, covering the cake with foil for the last 10 minutes if it is colouring too quickly. Remove the cake from the oven and test with a skewer – you want it to come out moist but clean. Leave the cake to cool a little in its tin before turning out on to a rack and peeling off the paper from its bottom.

To make the chocolate butter cream, snap the chocolate into small pieces and let it melt in a bowl balanced over a pan of simmering water (the water should not touch the bowl). Leave to melt, with little or no stirring, then add the butter, cut into small pieces, and the spice. Stir until the butter has melted. Leave to cool until the mixture is thick enough to spread. Spread the chocolate cream over the top of the cake and decorate as the whim takes you.