Lightly butter 8 ovenproof egg cups. Swirl a little caster sugar to coat evenly.
Cut the rhubarb into small chunks and place in a pan with the caster sugar. Cover the pan and cook over a low heat for 5 minutes, until softenend. Cool, drain the juices and finely chop the rhubarb.
Meanwhile, gently heat the custard, stir in the white chocolate and keep stirring until the chocolate melts, then cool. Beat in the egg yolks.
Preheat the oven to gas 6, 200°C, fan 180°C. When the rhubarb and custard mixtures have cooled, stir them together.
Whisk egg whites until they are stiff and fold into the rhubarb custard mix with a metal spoon, taking care not to knock much air out of the egg whites.
Spoon into the prepared egg cups to ½cm (¼in) below the top and bake for 8 to 10 minutes until soufflés are risen. Serve at once, dusted with icing sugar.