Heat the oven to 400°F/200°C. Line an 8 x 12-inch/20 x 30 sided baking sheet with parchment which you’ve buttered on both sides. Put the eggs and sugar in a glass or metal bowl, set it over a pot with an inch/2.5 cm of simmering water in the bottom (do not let the bowl touch the water) and beat until tripled in volume and very thick and ribbony, like cake batter. This will take about 10 minutes. Remove from the heat.
Sift the flour and nuts over the batter and gently fold together. Pour into the pan so that it is evenly distributed. Bake until slightly golden on top, 10 to 12 minutes. Meanwhile, get set up for unmoulding the cake: lay a damp tea towel on a cutting board or on the back of another baking sheet. When the cake comes out, flip it onto the towel, carefully peel off the paper, trim the edges it you want, and roll immediately. Set aside to cool to room temperature, about 5 minutes.
Whip the cream into stiff peaks with a hint of vanilla and a pinch of cinnamon. Unroll the cake from the towel, sieve over a dusting of cocoa powder, spread over the cream, and roll the cake up again. Set on a platter seam-side down. Sift over a little icing sugar and cocoa powder. Decorate with a button-row of hazelnuts. Cover and refrigerate until eating, which you’ll want to do the same day, preferably within a few hours. (Alternatively, save filling and rolling the cake until an hour or two before serving.)