Preheat the oven to 400°F / 200°C. Butter a 6in / 15cm soufflé dish and coat it with 25g of breadcrumbs.
Melt 50g of butter in a saucepan. Stir in 50g of all-purpose flour and 1 teaspoon of mustard powder, then gradually whisk in 300ml of milk. Cook, stirring, until thick.
Separate 4 eggs, putting the whites in a clean bowl. Stir the yolks and 100g of grated extra-sharp cheddar into the sauce.
Whisk the whites to soft peaks and gently fold them into the sauce. Pour into the dish and bake for 25-30 minutes, until golden and risen with a slight wobble.
