Boil the water and sugar in a 960-mL/1-qt sauce pot over high heat until the sugar is dissolved.
Turn off the heat and stir in the cocoa powder with a whisk until it is dissolved.
Return the pot to high heat and bring to a boil again; add the cream and reduce the heat to a simmer.
Cook until thickened, about 20 minutes; stir frequently. Add the bloomed gelatin. Pour 30g/1.06oz into each cavity (measuring 2.5cm/1in wide by 7.5cm/3in long by 3cm/1.25in deep) of a silicone rectangular mold and freeze. Reserve the remaining gel for later use. Discard after 1 week.
Once the gel has frozen hard, unmold the pieces and reserve frozen for service in an airtight container.