Pre-heat oven to 170 ºC.
Sieve the flour with the icing sugar and the cocoa powder, into a large bowl.
Chop the butter into the bowl or crumble by hand, until it looks like fine breadcrumbs.
Work in enough egg yolk to hold the crumbs together - volumes will depend on kitchen temperature and the water content of the butter.
Roll out on a floured board to fit a 25cm tart tin, with a removable base.
Prick the base with a fork and allow to rest in the fridge for 30 minutes.
Line pastry with foil, fill with dry beans, and bake blind for 15 minutes.
Remove the foil and beans, turn up heat to 180 ºC, and bake for a further 15 minutes to crisp the surface. Set aside to cool.
Melt the chocolate, butter and cream in a glass bowl over simmering water.
In another bowl, with a hand-held beater, whisk the eggs, extra yolk and the sugar, until fluffy.
Remove the chocolate mixture from the heat and whisk to blend. Set the egg bowl over the simmering water, and hand whisk until it starts to firm up a little (about 10 minutes).
Do not overheat, or the egg will scramble.
Remove from heat, fold in the chocolate, and pour into tart case. Don't overfill the case - use any leftovers as a sauce instead.
Bake at 180 ºC for 10 - 13 minutes, until the filling doesn't ripple any more. Don't panic if it wobbles a bit, it does firm up when it cools.
Sieve cocoa over cake, and serve warm with a dollop of cream and a cherry or edible flowers.
PS: This is well-known British writer Elizabeth Luard's chocolate tart recipe - how bad can it be?