For grapefruit sorbet, stir sugar, glucose and 180ml water in a saucepan over medium heat until sugar dissolves (3-5 minutes). Transfer to a bowl, add grapefruit juice, refrigerate to chill, freeze in an ice-cream machine, then freeze until required. Makes about 1 litre.
Meanwhile, cover grapefruit rind with cold water and bring to the boil in a saucepan over high heat. Drain, then repeat twice. Stir sugar and 40ml water in a saucepan over low heat until sugar dissolves, bring to the simmer, add rind and cook until candied and translucent (5-10 minutes). Drain (discard syrup) and set aside to cool on a piece of baking paper.
For chocolate gâteaux, preheat oven to 150C. Combine butter and chocolate in a heatproof bowl placed over simmering water, stir occasionally until melted (3-5 minutes), set aside. Meanwhile, whisk yolks and half the sugar in the bowl of an electric mixer until thick and pale (3-5 minutes), then fold in chocolate mixture and set aside. Whisk eggwhites in the bowl of an electric mixer until frothy, then, with motor running, gradually add remaining sugar and whisk until soft peaks form (3-5 minutes). Fold cocoa into chocolate mixture, fold in whisked cream, fold in whisked eggwhite and pour into eight 10cm-diameter springform tins buttered and lined with baking paper and placed on two oven trays. Bake, swapping trays halfway through cooking, until set (25-30 minutes). Set aside to cool in tin (30 minutes), then release sides and cool completely.
Meanwhile, for chocolate ganache, place chocolate in a heatproof bowl. Bring cream to the boil in a saucepan over high heat, pour over chocolate, set aside until melted, then stir until smooth and glossy. Set aside until slightly cooled and spreadable.
Remove gâteaux from tins, spread half with ganache and carefully sandwich with remaining gâteaux. Serve topped with grapefruit sorbet and candied rind.