1. Preheat the oven to 180°C (350°F). Line a baking tray with parchment paper and spread the hazelnuts on it. Bake for 10 minutes in the preheated oven. Let them cool slightly and rub them between your palms to remove the skins.
2. Coarsely grind the hazelnuts. In a bowl, sift the self-raising flour, cocoa powder, vanilla, and baking soda. Add the salt and mix.
3. In another bowl, combine the oil and sugar. Mix with a whisk until the mixture is light and fluffy. Add the vinegar and mix. Pour in the water and gradually add the flour mixture. Leave a little flour mixture on the surface of the batter and sprinkle the hazelnuts on top. This will help prevent them from sinking to the bottom of the cake.
4. Grease and dust a round bundt pan (a pan with a hole in the middle) with cocoa powder. Pour the batter in and bake in the preheated oven for 40 minutes. Let the cake cool, turn it out onto a plate, and decorate with hazelnuts.
