For the pastry, blitz 50 g hazelnuts in a food processor until finely chopped, then add 150g plain flour, 3 Tbs cocoa powder,
2 Tbs soft brown sugar and 150g cold butter cut into small cubes. Pulse together until the mixture looks like coarse sand,
then add a Tbs or two of cold water, pulse again until it comes together in a ball. Remove, wrap in cling film and chill for 15 minutes.

For the filling, beat together 5 egg yolks, 3 Tbs cocoa powder, 3 Tbs cornflour and 100g Nutella until smooth. Heat 150g soft brown sugar
and 250 ml double cream with a pinch of salt, until the sugar dissolves, then pour over the egg mixture, whisking until smooth over
a low heat until the mixture thickens. Pour into a clean bowl and add 100g finely chopped plain chocolate, stirring until melted and
smooth. Cover with cling film to prevent a skin forming, and leave to cool.

Roll out the chilled pastry and line a 23cm loose-bottomed tart tin. Scrunch up a piece of baking paper and use this to line
the pastry shell, then fill with baking beans and bake for 15 minutes in a 200°C oven. Remove the beans and paper and bake for
another 10 minutes, until the pastry is cooked. Let the pastry cool, then lightly whisk the chocolate custard and pour this into
the cooled pastry case. Smooth the surface and chill for several hours (or overnight).

For the meringue, beat 3 egg whites with 150 g caster sugar in a bowl over a pan of gently simmering water until the mixture has
thickened, then remove from the heat and whisk for another 6 minutes or so, until it forms stiff peaks. Spoon the meringue onto
the tart, making peaks with the back of the spoon, and then use a blowtorch to caramelise some of the meringue.