Mix flour and cocoa. Add butter in pieces, icing sugar, salt and egg. Knead first with the dough hooks of the hand mixer, then with your hands to a smooth dough. Cover and chill for about 30 minutes.
Place the nuts on a baking tray and roast in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for 5-10 minutes until golden brown. Remove and place on a tea towel. Rub hazelnut skin off with the help of the cloth. Coarsely chop nuts
Grease the tart mould with lift-off base (24 cm Ø) and dust with flour. Roll out the dough on a floured work surface (approx. 28 cm Ø). Line the mould with the dough, cut off any protruding edges. Prick the dough several times with a fork
Line the mould with baking paper and fill up with dried peas. Bake on the lower shelf in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-15 minutes. Remove dry peas and baking paper and bake for another 10 minutes. Remove the tart from the oven and let it cool down on a cake rack
Chop the chocolate finely. Heat the cream and remove from the heat. Melt the chocolate in the cream. Let the chocolate cream cool down a little, then fill into the tart. Chill the tarte for at least 4 hours. As soon as the chocolate cream begins to set, spread the nuts on top. Serve with crème fraîche
waiting time approx. 6 1/2 hours