Preheat oven to 175C. Heat brown sugar, 20gm butter and 1 tbsp water in a flame-proof 22cm x 12cm loaf pan over medium heat, stirring until sugar dissolves (1-2 minutes). Cut rhubarb into lengths to fit base of pan and place snugly beside each other over caramel.
Using an electric mixer, beat remaining butter, caster sugar and orange rind until pale and creamy, add egg yolks one at a time, beating well after each addition, add glacé ginger, stirring to combine. Sift dry ingredients together and add to egg mixture in 2 batches, alternating with half the orange juice. Beat eggwhites until stiff peaks form, add to batter in 2 batches and fold gently to combine. Spoon over rhubarb, level top and bake until a skewer withdraws clean (40-45 minutes). Stand for 10 minutes, turn out onto a platter and serve warm or at room temperature with mascarpone passed separately.