1) Sieve the flour, cocoa, salt and baking soda together in a medium bowl. Put the butter, icing and brown sugar in a food processor and process until smooth and creamy.
2) Add the sieved dry ingredients and beat to combine. Add the egg yolks and continue to beat to make a smooth dough.
3) Turn dough out onto a large sheet of clingfilm, form into a disc, and wrap tightly.
4) Refrigerate for between 1 hour and 2 days.
For the chocolate filling:
1) Put the chocolate pieces into a medium sized bowl. Put the cream and sweetened condensed milk in a small saucepan and bring to a simmer over medium heat. (Alternatively, heat the cream and milk in the microwave.) Pour the cream over the chocolate and whisk until smooth and shiny.
2) Whisk in the eggs and vanilla until smooth.
3) Cover, and set aside at room temperature.
To assemble the tart:
1) Place the dough between 2 pieces of waxed or greaseproof paper, roll into a circle about 0.5cm thick and refrigerate for 15 minutes. 2) Place a shelf in the centre of the oven and preheat to 200C/Gas 6. Peel one piece of the paper from the dough, then gently flip the dough into an 18cm loose-bottomed flan tin and peel away the rest of paper. Carefully press the dough evenly into the tin and trim off any excess dough from the edges.
3) Line the pastry shell with waxed paper or lightly sprayed greaseproof paper and fill with pie weights or ceramic baking beans.
4) Place the flan tin on a baking sheet and bake for 20 minutes. Remove the paper and weights and continue to bake for about 5 minutes more, until the crust sets and looks a little dull (a sign it is cooked). Reduce oven temperature to 160C/Gas 3.
5) Remove flan tin from the oven, spread the dulce de leche evenly over the warm crust and top with the chocolate filling.
6) Bake for about 25 to 30 minutes, until the filling sets around edges but is still slightly wobbly in the centre.
7) Cool completely and for a final flourish, sprinkle lightly with grey salt just before serving, if desired.
Busy Bakers' Tips:
Make the pastry dough and freeze it for up to a month in advance.
Make the pastry dough, bake it and then freeze, tightly wrapped, for up to 1 week.
Assemble and refrigerate up to 2 days, tightly wrapped.