Preheat the oven to 180 celsius (350 Fahrenheit) and grease and line a 22cm x 33cm (9 x 13 inch) [amazon_link id="B000TT7B86" target="_blank" ]swiss roll tin[/amazon_link] with baking paper.
Place the eggs, brown sugar and vanilla extract in a large bowl and whisk with an electric mixer until the mixture is extremely pale, thick and creamy, around 5 minutes. The mixture should look almost mousse like in consistency.
Add the oil and hazelnut liqueur (if using) and gently fold through with a metal spoon or spatula. Sift in the flour and salt and fold through gently, being careful not to knock too much air out of the whisked eggs. Once incorporated pour the mixture into the prepared tin and tilt to distribute evenly. Bake for 10 to 15 minutes, or until the cake has shrunk back a little from the sides and springs back when touched.
While the cake is in the oven dust baking paper generously with cocoa and when the cake is done remove it from the oven and immediately turn out onto the cocoa dusted baking paper. Remove the baking paper from the back of the cake and immediately, beginning at the narrow end of the cake, roll into a tight roll. Place on a [amazon_link id="B0001MS3DI" target="_blank" ]wire cooling rack[/amazon_link], seam side down, and leave to cool completely.
Carefully unroll the cake and spread the Nutella over the cake, making sure to go right to the edges. Sprinkle with 2/3 of the chopped hazelnuts and carefully re-roll. Lightly dust with extra cocoa and serve.
The cake is best eaten the day its made but will last 2 to 3 days in a covered container.