The pastry chef uses gelatin to make the glaze look mirror-like. The best cake when the occasion calls for a special cake that's not boring. And You Can Do It Too: Here' s the (Chocolate Glaze Mirror)
I used this mirror glaze recipe for Father's Day. It turned out great. It is sweet, so you might want to consider reducing the sugar by half. Also, there is a link to a webpage where there are written instructions for the cake and the mirror glaze.