Preheat your oven to 180 C /150 C fan and spread the hazelnuts out on a single layer on a large baking tray.
Roast for 12 minutes, shaking occasionally, until the skins turn crispy and start falling off.
Place the warm hazelnuts on a clean tea towel, fold the towel over and rub the hazelnuts vigorously. Most of the skins will fall off. Try and remove as much of the skin as you can, and leave the hazelnuts to cool.
Place the hazelnuts into the jug of a high speed blender and slowly blend for 5-8 minutes until the nuts start to turn into a creamy paste.
Break the chocolate into a heat proof bowl suspended over a pan of barely simmering water, and leave to melt, stirring occasionally.
Add the coconut oil, maple syrup, cocoa powder, vanilla and salt to the hazelnuts and blend to combine.
Pour in the melted, cooled, chocolate and blend until smooth. You can, at this stage, pour the mixture through a sieve to remove any pieces of hazelnut to make a smoother mixture.
Cover with cling film and leave overnight to solidify.
For the chocolate cake
Preheat your oven to 190 C/ 170 C fan and grease and line two 8 inch cake tins.
Sift the flour, sugars and bicarbonate of soda together in a large bowl.
Place the water, cocoa powder and butter together in a small saucepan over a medium heat and stir until the butter has melted. Do not boil. Remove from the heat and add the vanilla.
Add the vinegar to the milk to curdle it, and add the milk, the eggs and the melted chocolate mixture to the dry ingredients. Stir well to combine.
Spoon into the prepared tins and bake for 25-30 minutes, or until the cake is springy to touch and coming away from the sides of the tin. A skewer, inserted, will come out clean. Transfer to a wire rack to cool completely.
For the hazelnut brittle
Heat the sugar in a non-stick pan over a medium heat, stirring constantly in small slow circles with the handle of a wooden spoon or a chopstick.
When the sugar has melted and begins to turn brown, add the hazelnuts and stir well.
Pour the mixture out on a silicone baking mat and leave to harden.
For the chocolate hazelnut frosting
Spoon the failed chocolate hazelnut spread into a food processor and add the butter (cubed) and icing sugar.
Process until thoroughly combined, adding 3 or 4 tablespoons of milk to make a soft consistency.
Slice each of the cakes horizontally in half. Place the bottom layer on your cake stand and spread with 1/4 of the chocolate hazelnut frosting.
Repeat with the remaining layers, finishing with a nice thick layer of chocolate hazelnut frosting.
Break the hazelnut brittle into pieces and sprinkle over the top of the cake.
Mum of three, daydream adventurer, OU Life Sciences graduate, occasional crafter, ex-pat Canadian & quite possibly Britain's most northerly food blogger.