Firstly, prepare the crème patissière (it can be made up to one day in advance and kept refrigerated until required). Heat the milk with the vanilla pod in a heavy-based saucepan until almost boiling. Beat the egg yolks and sugar together in a large bowl, then stir in the cornflour.
Pour a splash of the hot milk into the egg mixture then whisk well until the mixture is smooth, then gradually whisk in the rest of the milk. Pour back into the pan and whisk over a medium-low heat for 3-5 minutes until thickened and smooth.
Transfer to a bowl, cover and cool to room temperature, stirring occasionally to prevent a skin forming.
Pre-heat the oven to 180°C / 350F/Gas Mark 4
Brush 6 deep individual ramekins with a generous layer of soft butter, using upward strokes and dust the insides either with sugar. Place the ramekins in the fridge for 5 minutes for the butter to set.
Blitz the blackberries with the lemon juice and icing sugar with a hand-blender then pass the puree through a sieve. Discard the seeds. This is your blackberry coulis.
Combine 250g of the berry puree into the crème patissière.
Now, in a clean bowl, whisk the egg whites to firm peaks, then gradually whisk in the 50g caster sugar a spoonful at a time to make a firm, glossy meringue. Whisk a third of the egg mixture into the crème patissière base, then very carefully fold in the rest, using a large metal spoon or spatula.
Fill each ramekin with the soufflé mix and smooth the tops with a palette knife, then run a little circle around the ramekin with your thumb.
Sit the ramekins on a wide baking tray and bake for 10-12 minutes until well risen and lightly golden on top. The soufflés should wobble gently in the middle when ready, dust with icing sugar and serve immediately with a serving of some of the retained coulis.