Luckily, Reddit user SuperDrew124 shared the secret behind this mirror-like glaze:

Ingredients: 20 g Gelatin Powder, 120 g Water, 300 g Glucose, 300 g Sugar, 150 g Water, 200 g Sweetened Cond Milk, 300 g Chocolate (White, Milk, Dark or a combination), Food Coloring

Steps:
1) Bloom the gelatin in the water;
2) Boil the glucose, sugar & water;
3) Remove from heat and add the gelatin;
4) Add the cond milk;
5) Pour over chocolate and beurre mix to remove air bubbles;
Use at 35C/95F.

The marbling effect is achieved by pouring different color glazes together over the cake.

-Bored Panda