Preheat the oven to 200°C, fan 180°C, gas 6. Place a baking tray on the top shelf of the oven to get really hot.
Melt 10g of the butter and use it to brush the base and sides of two 1-litre soufflé dishes, then coat them with the breadcrumbs.
Melt the remaining butter in a saucepan and stir in the flour, mustard powder and cayenne pepper. Cook for 1 minute, then remove from the heat and slowly add the milk, whisking all the time to prevent lumps. Return to a medium heat, stirring continuously until the sauce thickens.
Allow the sauce to bubble for 2 minutes – it will become very thick – then remove from the heat. Stir in the stilton, brie, cheddar and egg yolks, and season generously with plenty of freshly ground black pepper. The mixture should be about the consistency of Greek yogurt.
In a separate bowl, whisk the egg whites until you have medium to stiff peaks, then quickly mix 1 tbsp of the mixture into the cheese sauce. Carefully but quickly, fold the rest of the egg whites into the cheese sauce using a large metal spoon, trying to keep as much of the air in the mixture as possible.
Carefully pour the soufflé mixture equally into the prepared dishes, then run your finger around the mixture at the top of each dish – this will give a ‘top hat’ effect to the cooked soufflés.
Put both dishes on to the preheated baking tray, then cook in the oven for 20-25 minutes. Don’t open the oven door until the soufflés have cooked for at least 20 minutes. If they still wobble a lot, leave them in the oven for another 5 minutes. Serve immediately.