To serve: cream, ice-cream or thick Greek style yoghurt
Instructions
For the pastry, place cocoa and icing sugar in a food processor and process until free from lumps. Add flour and process until combined. Add butter and process gently or pulse until the mixture resembles coarse breadcrumbs. Add the sour cream and pulse again just until it forms a ball. Wrap in plastic wrap and refrigerate for at least 20 minutes.
Grease a 35cm x 13cm rectangular flan tin with a removable base with butter. Roll out the dough between two pieces of baking paper to 5mm thick. If the dough is too sticky, dust it with sifted cocoa. Using the baking paper to lift the pastry,  carefully lift it into the prepared tin, ensuring the pastry overlaps the edges. Alternatively, if you find it too hard to handle, push the pastry into the tin with your fingers. Tuck it into the corners and trim neatly by running a rolling pin over the edge. Use any excess to patch any thinner parts or holes.
Pre-heat oven to 200°C. Line pastry with baking paper and pastry weights or dried beans and blind bake for 25 minutes, then remove paper and weights and return to oven for another 5 – 10 minutes or until dried out. Make sure the pastry is cooked as the tart is not cooked again.
For the ganache: heat cream, butter and broken or chopped chocolate in a small saucepan and stir until melted. Stir in Cointreau. (If you are unsure if the chocolate is all melted, sieve.) Pour immediately  into baked tart shell in one pour.  Leave to set. Do not refrigerate. If you refrigerate over night, bring to room temperature to eat.
To serve place long knife in boiling water and cut small pieces as the tart is very rich. Serve with cream, ice-cream or thick Greek style yoghurt.
