Start by making the cake. Grease a 10cm (4in) tin can or deep cake tin and line the base and sides with greaseproof paper. Heat the oven to 180C/350F/gas mark 4.

Mix together the cocoa powder and boiling water until it forms a smooth paste. Add all of the other ingredients and whisk well with an electric mixer until smooth. Pour the batter into the tin and bake for about 30 minutes until a skewer inserted into the centre of the cake comes out clean. Leave to cool in the tin on a wire rack until just warm, then turn the cake out onto the wire rack and leave to cool completely.

To make the ganache, place the chocolate in a heatproof bowl, heat the cream until almost boiling then pour it over the chocolate, leave for a couple of minutes then add the nutella and stir until smooth. Set the ganache aside until it has thickened but is still spreadable, you can put it in the fridge to speed this up, but do keep an eye on it so that it doesn't set too hard.

To make the hazelnut praline and decorative spikes, heat the oven to 180C/350F/gas mark 4. Place the hazelnuts on a baking sheet in a single layer and toast in the oven until the skins have blackened and the nuts are golden. Tip them out onto a clean tea towel, wrap them up in it and then give them a rub - the skins should flake off, don't worry if you can't get rid of it all though.

Take about 10 of the hazelnuts and stick a cocktail stick into each one, place a sheet of baking parchment directly under your worksurface to catch any drips of caramel.

Place the sugar in a small, heavy bottomed saucepan and heat until the sugar has melted and caramelised, swirl the pan a little so that the sugar melts evenly, but don't stir it. Remove from the heat and allow the caramel to thicken to the consistency of golden syrup.

Dredge each hazelnut through the thickened caramel, coating it generously. Secure the cocktail stick to the work surface with some sellotape or wedge under a heavy pan, so that the caramel covered hazelnut hangs over the edge. Allow the caramel to drip, like a stalactite, towards the floor and set hard. Repeat with all the hazelnuts and allow the caramel to set. The caramel in the pan sets quickly, so you will probably need to re-melt it over a low heat a couple of times.

When hardened, use scissors to gently trim the fine threads of caramel off the ends of the spikes; return the trimmings to the pan, along with any caramel that has collected on the sheet of greaseproof paper. Re-melt the pan of caramel then stir in the remaining hazelnuts and pour it out onto a tray lined with greaseproof paper and allow to set and cool.

Place the cooled praline into a tough ziplock bag and seal it then bash with a rolling pin to break it up into small pieces.

To assemble the cake, trim the top off the cake to level it, then slice it into three even layers. Place one layer on a serving plate and spread over one third of the thickened ganache, sprinkle over some of the praline and place another cake layer on top. Repeat until you have used all of the cake layers and ganche. Remove the cocktail sticks from the hazelnut spikes and use the best looking ones to decorate the top of the cake along with more praline.

The cake is best served shortly after decorating as the moisture in the ganache will cause the caramel to melt after a while, also for this reason do not refrigerate the cake once it has been assembled.