Preheat the oven to 200C/400F/Gas 6.
For the rhubarb, melt the butter in a small ovenproof frying pan over a medium heat. Add the rhubarb, caster sugar and cardamom pods and cook down until soft.
To make the sponge, place the butter and sugar in a food processor and blend together.
Add the eggs, flour and milk and blend until the mixture is smooth.
Pour the sponge mixture onto the rhubarb in the pan and spread evenly. Transfer to the oven and bake for 12 minutes, or until the sponge is risen and golden.
To serve, turn the pudding out onto a warm plate and dust with the icing sugar.