Put the butter, sugar, eggs ... Spoon the cake batter into the prepared tin. Top with the rhubarb, pressing it down gently. Sprinkle with the reserved 3 tablespoons of sugar.
For the rhubarb, put the butter, sugar and five tablespoons of water in a large frying pan with the orange zest and rhubarb. Place over a gentle heat to poach for 4-5 minutes, or until just tender ...