To make the base, sift the flour, add the cubed butter and use your fingertips to rub the flour and butter together until it resembles breadcrumbs.
Add the sugar, and mix well.
Crack in the egg, and knead gently to get a dough.
Shape it into a ball, cover with clingfilm, and refrigerate for at least 30 minutes.
Preheat the oven to 180 degrees Celsius (350 Fahrenheit).
Roll the dough and transfer it to a pastry case, using the rolling pin to cut the edges so that the pastry can fit the case.
Cover the pastry with kitchen foil, and fill the case with baking beans or pulses (blind baking).
Bake for 10 minutes, then remove the foil and beans, and continue to bake for a further 10-15 minutes or until the pastry is golden.
Remove from the oven, and leave to cool completely.
To make the filling, use a heat-proof bowl, break the chocolate into pieces, add the butter cut into cubes and the double cream.
Place the bowl over a pan of simmering water, and cover the bowl with a plate, so that the chocolate can melt completely.
Give it a good stir, until you get a smooth filling.
Pour the filling over the pastry, and leave to set either at room temperature for a few hours, int he fridge for 2-3 hours, or in the freezer for 1 hour.