Prepare 8 in. souffle dish, by greasing and dusting lightly with extra sugar; place a greased paper collar around outside of dish, secure firmly with string.
Rub chestnut puree through sieve, place in saucepan with butter and sugar.
Stir constantly over low heat until butter is melted and sugar dissolved; add vanilla.
Remove from heat, cool slightly.
Beat egg-yolks until thick and creamy, gradually add chestnut mixture, beating constantly.
Beat egg-whites until soft peaks form.
Lightly fold half the egg-whites into chestnut mixture, then remaining egg-whites.
Pour mixture carefully into prepared dish, sprinkle top lightly with extra castor sugar.
Bake in moderate oven, Mark 4, 350°F. 50 to 60 minutes.