Sift the flour, baking powder and 2 level tablespoons cocoa powder into a bowl. Stir in the sugar, vanilla extract and fresh cream, then beat until smooth. Spread the mixture into a buttered 1.3L ovenproof dish.
Mix the demerara sugar with the remaining cocoa powder and sprinkle over the mixture in the dish.
Pour 350ml boiling water all over the pudding.
Bake in the preheated oven until risen and the sponge mixture (which will have come to the top) feels lightly firm to the touch, about 50 minutes.
Turn out and serve with fresh whipped cream or dairy ice cream.