Short answer: a well-emulsified ganache, a crumb coat, and patience while it cools to the right consistency.
A glassy ganache finish is mostly about temperature and preparation. Make the ganache smooth first: pour warm (not boiling) cream over chopped chocolate, wait a moment, then stir from the centre out until glossy — no stirring in air. Give the cake acrumb coat and chill it so the surface is firm and clean. Let the ganache cool to a thick, spreadable (or just-pourable, for a poured finish) consistency before it goes on — too warm and it slides off, too cool and it drags. A warm palette knife smooths the sides beautifully.
Related:chocolate celebration cake · glossary:ganache,crumb coat.