Preheat the oven to 180°C.
Line a 24cm spring-form cake tin with baking parchment.
Melt the chocolate and butter in a bowl over a pan of boiling water.
Whisk egg yolks with 30ml of sugar until light.
Whisk egg whites in a separate bowl until firm peaks form with remaining cake sugar.
Combine the melted chocolate with the egg yolk mixture.
Fold a third of the egg whites into the chocolate mixture to lighten it.
Gently fold in the remaining egg whites until fully incorporated.
Bake the mixture in the tin for 20 minutes until just set and leave to cool.
Place rhubarb on a tray and scatter with the remaining sugar.
Pour the Pimms over the rhubarb and roast for 15 minutes until tender.
Serve cake slices with roasted rhubarb and a scoop of ice cream.