Preheat the oven to 350°F, gas mark 4, 180°C (160°C fan-assisted).
Using three 20cm sandwich tins, line the base of each tin with parchment paper.
Place a medium glass bowl over a saucepan filled with simmering water on the stove and melt the coconut oil and 250g of the dark chocolate (put aside 85g of chocolate, chunks for later). When melted stir and remove from the heat.
Using an electric whisk, in a clean bowl beat the egg whites until thick and glossy, and set aside.
In a large mixing bowl, add the egg yolks and beat with a fork until mixed. When the chocolate mixture has cooled, add to the egg yolks and stir until smooth. Then add the coconut sugar, coconut flour, vanilla, baking powder, tahini and sea salt and ensure its thoroughly mixed.
Gradually fold in the egg whites, lightly, to ensure you incorporate the air to keep the mixture light and fluffy. Once the egg whites are thoroughly folded into the mixture, fold in the 85g chocolate chunks.
Divide the mixture into the pre-lined sandwich tins and bake in the pre-heated oven for 20-25 minutes. Remove from the oven and leave to cool in the tin for 5-10 minutes. Use a knife to run around the edge of the tin to ensure that you can remove the cake from the tin when cooled.
When the cakes have cooled slightly, remove from the tin and place on a cooling rack, until completely cooled (you can make the cake the day before and cover each cake with foil on separate plates)
To make the avocado chocolate cream-
In a food processor blend the avocadoes until smooth, occasionally wiping down the sides of the processor using a spatula. Add the coco or cacao powder, agave nectar and sea salt. Blend until the mixture is completely smooth, again ensure the sides of the food processor are wiped down so that the mixture has no traces of avocado.
To assemble the cake, using a plate or cake stand, remove the parchment paper from the cakes, place the first tier of the cake down and fill with the avocado chocolate cream, then repeat with the other two layers (ensure that you leave enough of the mixture to cover the entire cake, sides and top). With the remaining avocado chocolate cream, cover the sides and top the cake and smooth out.
Scatter the halva or nougat over the top of the cake, or alternatively grate a little dark chocolate.