To make the pastry, sift the flour, cocoa and icing sugar into a bowl. Rub in the butter until it resembles fine breadcrumbs and stir in a pinch of sea salt.
Stir in the egg yolk and 45ml water, then bring together into a dough, add more flour if necessary. Wrap in clingfilm and leave to rest in the fridge for 30 minutes.
Preheat the oven to 190°C/375°F/gas 5.
Line a 25cm fluted tart tin with the pastry, pressing into the sides. Line with baking paper and fill with baking beads or rice. Set on a baking sheet and rest in the fridge for 30 minutes.
Transfer the tin to the oven on the baking sheet for 15 minutes. Roast the nuts for the brittle at the same time, until they are golden brown. Remove the baking paper and beads or rice. Return the pastry to the oven for 5 minutes, or until the base has cooked through. Set aside.
To make the filling, place the sugar and honey into a pan and set over a medium heat. Once the sugar begins to melt, swirl in circular motions to start the caramel until it’s golden. Remove from the heat and whisk in the cream and butter. Place back onto the heat and whisk until the caramel turns golden brown. Remove and pour into the tart case, then set aside.
To make the nut brittle, line a baking tray with baking paper. Put the sugar in a pan with the thermometer and set over a medium heat. Have your roasted hazelnuts ready.
When the sugar starts to gently bubble at the edges, tilt the pan to ensure the sugar melts evenly. Before long (about 3 minutes) all of it will be melted. When it gets to 150°C, remove the pan from the heat and swirl in the hazelnuts (do not stir with a spoon). Quickly pour it onto the tray and leave to aside.
When the nut brittle is cool, chop or blitz in a processor to small pieces. Sprinkle the pieces of brittle over the top of the finished tart.
To decorate with spun sugar, heat the sugar and glucose with 50ml water in the same pan, over a medium heat. Allow the sugar to melt, stir for even texture. Heat for 8 to 10 minutes, until 150°C, then remove and quickly place into an ice bath to stop the process and set aside.
Take two forks and pull the sugar out, twirling the forks to make long strands into nests. Alternatively, place a piece of baking paper on your countertop. While holding a long metal spoon with one hand over the paper, pull the sugar from the pan and quickly flick it back and forth over the long spoon, allowing the sugar to form long thin strands, which you can gather. Use these to decorate the top of the tart.