Preheat the oven to 200°C, fan 180°C, gas 6. Using a pastry brush, lightly brush the insides of 6 ramekins with the 30g melted butter, using upward strokes. Coat with a little caster sugar, then place on a baking tray.
Melt the remaining 50g butter in a small saucepan over a medium heat. Add the flour and stir with a wooden spoon until smooth and starting to bubble. Gradually pour the milk into the pan and continue stirring until the mixture is thick and smooth. Allow the sauce to simmer on a low heat for about 2 minutes, then remove from the heat.
Tip in the chocolate and remaining 3 tablespoons caster sugar, and mix until melted and combined. Add the egg yolks, one at a time, beating well between each addition. Transfer to a medium bowl. Using an electric hand mixer, whisk the egg whites in a bowl until soft peaks are formed. Tip half the egg whites over the chocolate mixture. Using a metal spoon, lift and fold the whites through the mixture.
Add the remaining egg whites and fold through. The mixture should look spongy and frothy. Spoon into the ramekins to just below the rim. Pop the tray in the oven and bake for 15 minutes. Resist opening the oven or they'll sink – they'll be ready when the centre is well risen. When pressed, they should yield but not feel liquid-like. Dust with icing sugar and serve immediately.