In a small saucepan over medium heat, whisk together the heavy cream, water, cocoa powder, and sugar-free chocolate syrup.
Bring to a gentle boil, stirring constantly.
Reduce the heat and simmer for 1 minute.
Remove from the heat, stir in the vanilla extract, and pour the mixture evenly into two ice cube trays.
Freeze the trays for at least 3 hours, or until the cubes are completely solid.
When ready to serve, empty the frozen chocolate cubes into a blender. Pulse until the cubes are broken up, then blend until you reach a smooth, slushy consistency.
Serve immediately.
Freeze the cubes in a zip-top bag after they're solid. This way, you can have a single-serving slushy ready to blend in minutes.
Garnish with a dollop of sugar-free whipped cream and a dusting of cocoa powder for a truly decadent experience.