Make chocolate curls by paring the underside of the block of chocolate with a swivel-bladed vegetable peeler. If the curls are very small, microwave the chocolate in 10-second bursts on full power until the chocolate is soft enough to shape. When you have enough curls to decorate 6 mousses, set aside, then break the remaining chocolate (200 g/7 oz) into pieces.
Melt the chocolate pieces in a heatproof bowl set over a saucepan of gently simmering water, making sure the bottom of the bowl does not touch the water. Stir the butter and glucose into the melted chocolate, then mix in the orange juice. Stir in the egg yolks one at a time until the mixture is smooth. Remove the bowl from the pan and leave to cool.
Whisk the egg whites in a clean bowl until soft peaks form. Whip the cream in a separate bowl until it forms soft swirls. Fold the cream, then the egg whites, into the chocolate mixture.
Pour the mixture into 6 freezer-proof coffee cups or ramekin dishes. Freeze for 4 hours or overnight until firm. Serve decorated with the chocolate curls.