Pile the flour on clean work surface, mix with sugar, cocoa and salt and make a well in the center. Cut the cold butter into small pieces, then put in the well of the flour with the egg and approximately 30 ml (approximately 1 ounce) of lukewarm water. Cut all ingredients with a knife to combine until small crumbles form, then knead with hands quickly into a smooth dough. Form into a ball, wrap in plastic wrap and chill in refrigerator for 30 minutes.
Roll out the dough on lightly floured surface and place in the greased tart mold. Cut parchment paper to fit the tart pan, place on dough, then fill with dried beans. Blind bake in a preheated 200°C (approximately 400°F) oven for about 20 minutes. Take from oven, let cool slightly, then remove beans and baking paper and set aside.
Melt the chocolate in a double-boiler over hot not boiling water. Heat the crème fraîche in a saucepan and stir in the dark chocolate, then slowly stir in the softened butter. Pour into the tart pan, smooth out and let cool completely. Garnish with fresh whipped cream and a sprinkling of cocoa. Serve immediately.