Preheat the oven to 150C/300F/Gas 2. Grease a 20cm/8in baking dish with butter and dust the inside with a little caster sugar
Heat a frying pan and toast the breadcrumbs until golden-brown. Remove the pan from the heat and the breadcrumbs to the brown sugar, mixing until well combined..
Heat the milk, golden caster sugar, lemon zest and butter in a saucepan until the butter begins to melt.
Stir in the breadcrumbs and set aside to cool.
Beat in the egg yolks and pour the mixture into the prepared baking dish. Cook for 25-30 minutes, or until golden-brown on top. Remove the dish from the oven and increase the oven temperature to 200C/400F/Gas 6.
Place the rhubarb in a saucepan with 200g/7oz of the sugar. Heat until simmering, then set aside to cool. Spoon the rhubarb on top of the cake mixture.
Meanwhile, mix 1 tbsp warm water with the jam and spoon on top of the rhubarb.
Finally, whisk the egg whites in a bowl until soft peaks form when the whisk is removed. Slowly whisk in the three tablespoons of sugar until stiff peaks form when the whisk is removed. Spoon the mixture on top of the pudding and bake for a further 8-10 minutes, or until golden-brown.
Serve with lightly-whipped cream.