Warm the milk and butter until the butter melts. Add the chocolate ( use best eating dark chcolate of at least 70% coca solids )and stir over gentle heat until that has melted.
Fold in the breadcrumbs and vanilla ( if used - optional ) and stir over gentle heat 5 - 10 minutes until smooth and glossy, with all the oiliness absorbed.
Set aside until it has cooled to hand hot, then stir in the egg yolks and sugar until smooth again.
Beat the egg whites until thick, then fold them in very gently, about ¼ at a time.
Lightly butter a pudding basin, put a disc of buttered greaseproof paper in the bottom of it to prevent sticking, and gently pour in the mixture.
Cover basin tightly with foil, steam or place in a covered pan of hot water at a gentle boil for 2 - 2½ hours. A knife inserted in the centre should be slightly coated with gooey cakey mix for perfection.