Cream the butter and sugar in a mixer. In a bowl, combine flour and cocoa. Add to the butter mixture. Add egg yolks and beat to form a dough. Wrap the dough in plastic wrap and refrigerate 1 hour.
Preheat oven to 180 degrees C. Roll out the dough on a floured surface and place in an oiled tart tin with a removable base. Bake 10 minutes and set aside.
Place the chocolate in a bowl. In a saucepan, melt the butter and sugar and heat to boiling. Pour the hot liquid over the chocolate and let stand 3 minutes, then stir until smooth. Add the eggs one at a time, stirring constantly so the eggs don't cook.
Pour into the tart base and cook another 10 minutes or until centre is set.
In a bowl, combine chocolate and syrup. Bring the cream to a simmer and add to the chocolate mixture. Add the butter and stir, always in the same direction so as not to cause bubbles.
Pour the topping over the filling. Let stand 15 minutes then sprinkle with fleur de sel. Serve at once.