50g ground hazelnuts 1 tbsp caster sugar for the filling 100g hazelnuts 80g walnuts 75g caster sugar 250g fine dark chocolate 40g butter 25ml espresso coffee 4 eggs For the pastry: put the flour and butter into a food processor, add a pinch of salt and blitz to coarse breadcrumbs (if you prefer, rub the butter into the flour with your fingertips). Add the ground hazelnuts, egg yolk, sugar and enough water to bring the dough to a firm ball, as above. The less water you add the better. Turn the pastry out on to a heavily floured board and knead very lightly. Pat into a flat round, then roll out to a circle large enough to line a 24cm tart tin that is at least 3.5cm deep. Push the dough right into the corner where the rim joins the base, without stretching it. The pastry is rich and very fragile, so push it into place, patching up any holes as you go. Trim the overhanging pastry and then place in the fridge to chill for 20 minutes. Set the oven at 200C/gas mark 6. Put a baking-sheet in the oven to heat. Line the pastry case with kitchen foil, fill with baking-beans and slide it on to the hot baking-sheet. Bake for 15 minutes, then remove from the oven and carefully lift out the beans and foil. Return the pastry case to the oven for five minutes or so, until the surface is dry to the touch. Remove from the oven and set aside. For the filling: toast the hazelnuts and walnuts in a shallow pan until the skins have darkened. Put them in a tea towel and rub roughly until the skins flake off. Return the nuts to the pan and continue toasting until they are light brown and deeply fragrant. Put the sugar into the pan with the nuts, place it over a moderate heat and leave until the sugar starts to melt. Once the sugar turns golden, tip the mixture on to a lightly oiled baking-sheet and leave to cool. Snap the chocolate into small pieces and put it, with the butter and the coffee, in a mixing-bowl. Place over a saucepan of simmering water and leave it to melt. The less you do to the chocolate the better, so avoid the temptation to stir it more than once or twice. Meanwhile, crush the sugared nuts with a knife or in a food processor - they should be a mixture of textures, from fine to coarse gravel. Remove the melted chocolate from the heat. Separate the eggs, lightly stirring each yolk into the chocolate. In a large bowl, beat the whites until they are thick and fluffy. Fold the egg whites into the chocolate. This is easiest with a large metal spoon. Make sure that you leave no bits of egg white unmixed, at the same time taking care to treat the mixture gently. Tenderly fold in the crushed nuts, trying not to knock out the air in the process. Scoop the filling into the cooled tart case, then place in the fridge for two or three hours to set. The filling will be at its best after this time. Any longer may make the pastry damp.