Preheat the oven to 200C/400F/Gas 6. Grease three dariole moulds with butter and dust with caster sugar, shaking out any excess.
Whisk the egg whites and lemon juice in a clean bowl until soft peaks form when the whisk is removed.
In another bowl, beat the arrowroot with the egg yolks and then whisk these into the whites. Fold the orange zest and juice into the mixture. Spoon into the prepared dariole moulds.
Dust the puddings with cocoa powder and bake in the oven for 5-8 minutes, or until risen and set. Serve immediately.
