In a medium bowl, bloom gelatin in cold water for about five minutes ("blooming" is the process of soaking gelatin in water before using it). Gently squeeze excess water from gelatin.
In a small saucepan over medium-high heat, stir glucose with water until the glucose is dissolved. Add cream and cocoa and bring to a simmer, stirring to combine. Remove from heat, add gelatin and stir to dissolve.
Pour mixture through a fine mesh strainer and allow to cool to 35°C/95°F, stirring gently and occasionally to prevent skin from forming.
Once cooled, pour over a well-frozen, masked cake ("masking" is the process of putting a thin coat of icing over a cake so that crumbs don't get into the icing).
Allow to set in fridge for 15 minutes before cutting to serve.