Preheat the oven to 180°C/gas mark 4 and grease and line a shallow tin with baking paper.
Whisk the eggs and sugar until they are pale and light and have roughly trebled in volume. Fold the flour in carefully, trying to keep as much air in the mixture as possible.
Pour the batter into the prepared tin and spread to the edges.
Bake for approximately 8 minutes, until lightly golden and springy to the touch.
Sprinkle icing sugar over a large piece of baking paper and turn the sponge out onto it. Roll up lightly and leave to cool whilst making the filling.
Cut the rhubarb into 2-3cm pieces and place in an ovenproof dish. Sprinkle over the sugar and seeds from a split vanilla pod. Bake in the oven for approximately 20-25 minutes, until tender, leave to cool.
Drain the juice from the rhubarb and pat dry on kitchen paper.
Unroll the sponge. Tear the rhubarb into strips and place on the sponge.
Whisk the cream until it forms stiff peaks. Fold in the custard and mix well. Spread the custard mixture over the rhubarb.
Roll up the roulade as tightly as possible. Cover and place in the fridge until serving. Sprinkle with more icing sugar before serving.