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, sugar and vanilla extract in a large bowl and whisk with an electric mixer until the mixture is extremely pale and creamy, around 5 minutes.
Add the oil and milk and gently fold through with a metal spoon or spatula. Sift in the flour, cocoa and salt and fold through gently, being careful not to knock too much air out of the whisked eggs.
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 with cocoa and when the cake is done remove 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, to cool completely.
To make the Italian buttercream place the sugar, golden syrup and water in a saucepan and mix until well combined. Use a pastry brush to dislodge any stray grains of sugar that have stuck to the side of the saucepan before placing over a high heat. Bring to a rapid boil and add a [amazon_link id="B001FB6IFY" target="_blank" ]candy thermometer[/amazon_link]. Do not stir! Cook until the mixture reaches 160 celsius (320 Fahrenheit) and remove from the heat.
Place the egg white in the bowl of an [amazon_link id="B00005UP2P" target="_blank" ]electric mixer[/amazon_link] and whisk to soft peaks. With the mixer running, slowly pour the sugar syrup into the bowl, being careful not to touch the whisk, and continue whisking until the mixture cools, around 5 to 10 minutes. Add the butter, one third at a time, and whip until fully incorporated.
Carefully unroll the cake and spread the buttercream over the chocolate roulade and re-roll. Lightly dust with extra cocoa.