1. Separate egg yolks for the egg whites.
2. Using a blender, beat egg yolks with sugar and vegetable oil until creamy.
3. Whisk egg whites until stiff peaks form.
4. Fold egg whites into the yolk mixture carefully, then sift in the flour and fold it into the batter too.
5. Divide the batter into 2 portions and mix one with cocoa powder and another with cornstarch.
6. Pour the white half of the batter onto a parchment-covered baking sheet, and spread evenly.
7. Bake for 5 minutes at 190°C/375°F.
8. Pour the chocolate batter on top of the base layer and spread it out evenly. Bake for 8 minutes.
9. Let the roll cool down, peel the parchment back, and spread hazelnut cream on top of the cake base. Roll tightly and leave in the fridge to set.
10. Carefully slice the roll and dip each slice in melted chocolate, covering one side.