For the pastry:
1 Sift the flour, cocoa powder, and salt together into a large bowl and rub the butter into the mix until it resembles fine breadcrumbs.

2 Add the icing sugar, followed by the egg, and continue to rub together until all the ingredients are combined. Wrap the dough in clingfilm and place in the fridge for at least 30 minutes to chill.

3 Preheat the oven to 200 degrees (180 degrees fan) and grease a 24cm loose-bottomed fluted tart tin.

4 Roll out the dough on a lightly floured surface until you have a disc that is slightly larger than your tart tin. Press the pastry into the tart tin, leaving a little pastry hanging over the edge. Prick the base of the tart with a fork and place the tin in the freezer for five minutes.

5 Bake the tart blind by lining with a large square of parchment paper and filling with baking beans and baking in the oven for 15-20 minutes.

6 Remove the beans and paper and bake for a further five minutes. Remove from the oven and allow to cool slightly. Trim off the excess pastry and set aside.

For the caramel custard:
1 Put the gelatine leaf in a bowl of cold water to soak.

2 Put the sugar in a medium saucepan and place over a medium heat. Cook until the caramel becomes a deep golden colour, swirling the pan regularly to ensure it colours evenly.

3 Once the caramel has reached a dark golden brown, reduce the heat and add the cream little by little, whisking all the time, taking care, as the caramel will spit.

4 Bring the caramel back to the boil and add the vanilla. Pour this over the egg yolks then pour it all back into the pan, continuing to cook over a medium heat for about two to three minutes until the caramel coats the back of a spoon. Squeeze the water out of the gelatine leaf and add to the pan. Whisk to dissolve and cool slightly. Slowly pour into the tart base. Leave to set for at least 30 minutes.

For the chocolate layer:
1 Soak the gelatine leaf in cold water. Heat the cream and milk to a simmer in a saucepan over a medium heat. Meanwhile, whisk the egg yolks and sugar together in a bowl.

2 Once the cream is simmering, pour it over the egg and sugar, then pour back into the pan. Cook for a minute or two until the mix coats the back of a spoon.

3 Remove from the heat and add the gelatine, with the water squeezed out, then pour this over the chocolate and stir to melt the chocolate completely. Let it cool slightly, then pour this over the set caramel.

4 Chill for at least an hour until set.