Making soufflés is easy when you have Mary to guide you – and this huge chocolate version is a sure fire hit.
Preheat the oven to 190°C/170°C fan/Gas 5 and place a heavy baking sheet on the middle shelf of the oven.
Grease a 14cm x 8cm (1.2 litre) souffle dish with butter and dust with caster sugar. Add a 7.5cm collar of parchment paper folded at least 4 times and secured with string.
For the crème pâtissière chocolat, melt the chocolate in a bowl set over a pan of simmering water. Remove from the heat and leave to cool slightly.
Pour the milk and cocoa powder into a heavy-based pan and bring gradually to the boil. Remove from the heat and leave to cool for 30 seconds. Meanwhile, in a medium bowl, whisk together the egg yolks and caster sugar until pale, then whisk in the plain flour. Pour the chocolate milk onto the eggs, whisking continuously, then pour back into the pan.
Bring back to the boil, whisking continuously over a medium heat and cook for 1 minute until thickened. Remove from the heat and whisk in the melted chocolate. Spoon the crème pâtissière chocolat into a bowl, cover the surface with clingfilm to prevent a skin forming and leave to cool to room temperature.
In a clean bowl, whisk the egg whites to soft peaks. Add the sugar and continue whisking to a stiff and glossy meringue.
Whisk one third of the meringue mixture into the crème patissiere chocolat, then fold in the remaining two thirds with a large metal spoon. The mixture should be fairly loose.
Pour the mixture into the souffle dish. Run your thumb around the interior rim of the dish to ensure a good rise and bake on the baking sheet in the pre-heated oven for 40–50 minutes until well risen and springy to touch.