Sprinkle the gelatin over the cold water in a small bowl. Stir gently and let it bloom for 5 minutes until it swells and becomes spongy.
In a medium saucepan, combine the heavy cream, powdered erythritol, instant espresso powder, and cocoa powder. Warm over medium heat, whisking frequently, until the mixture is hot and steaming with everything fully dissolved. Do not let it boil.
Remove the pan from the heat. Add the bloomed gelatin and whisk until it melts completely into the warm cream, about 1 minute. Stir in the vanilla extract.
Pour the mixture through a fine mesh sieve into a spouted measuring cup or pitcher for easy pouring. Divide evenly among 4 ramekins or dessert glasses.
Let the panna cotta cool to room temperature for about 20 minutes, then cover each with plastic wrap and refrigerate for at least 4 hours or overnight until firmly set.
Before serving, whip the remaining heavy cream with powdered erythritol until soft peaks form. Top each panna cotta with a dollop of whipped cream and a light dusting of cocoa powder.