Preheat the oven to 180°C, gas mark 4 and line a large baking tray or Swiss roll tin (about 32x22cm) with baking parchment. Using a stand mixer, whisk the eggs and sugar together for about 8 minutes, or until they are thick and frothy and the whisk leaves a trail that is visible for at least 3 seconds when it is removed.
Sieve over the flour and cocoa powder. Use a balloon whisk to gently fold the dry ingredients into the foam until no floury patches remain. Scrape the mixture into the lined tin and use a spatula to spread it right into the corners.
Bake for 10-12 minutes. The cake should be risen and firm to the touch. Remove from the oven and carefully flip the cake onto a sheet of baking parchment dusted with icing sugar. Peel the baking paper off the bottom of the cake, then roll tightly, starting at the short edge. Leave to cool completely while rolled up.
To make the buttercream filling, beat the butter in a large mixing bowl with an electric whisk until loose. Add the icing sugar in stages, whisking until smooth, then add the double cream. Beat for a few minutes more until light.
Bring a small saucepan of water to a gentle simmer. Place the chocolate, butter and syrup for the icing into a large, heatproof bowl. Without the bowl touching the water, set it over the top of the pan. Stir until melted, then remove from the heat and add the cream. Cool, then chill the ganache for at least 30 minutes, or until ready to use.
When the cake is cool, carefully unroll it. Don’t worry if it cracks, because it will be coated in ganache. Using a palette knife, spread an even layer of buttercream over the sponge, then spread with hazelnut spread and roll it up again. Pipe ganache over the cake, sprinkle with hazelnuts and serve.