Pour the cold water in a small bowl and sprinkle the powdered gelatin over it. Allow it to set for 5 minutes.
Add the half and half and sugar to a medium pot over medium-low heat, stirring occasionally until steaming and hot. The sugar should completely dissolve.
Remove the pot from the heat and add the gelatin mixture, whiskey, salt and espresso powder. Stir until the gelatin and espresso powder have dissolved and the mixture is smooth.
Pour the mixture into 4 large or 6 small dessert glasses or ramekins until they are ¾ full. Place the glasses in the fridge to set for 1 hour (they will be soft set, but not completely set).
In a small pot, add ¼ cup of the heavy cream and all of the Irish cream. Sprinkle the gelatin over the liquid and let set for 5 minutes.
Add the sugar and salt and mix until everything is combined (it's okay if the mixture is lumpy at this stage).
Heat until the mixture is hot, and the gelatin and sugar are dissolved. Remove from heat and set aside.
Use a stand mixer or hand mixer to whip the other ½ cup of heavy cream until soft peaks form.
Fold the Irish cream mixture into the bowl with the heavy cream until combined.
Spoon the Irish cream mixture over the set coffee layer to almost the top of your glasses or ramekins. Refrigerate for at least another 3 hours, until completely set.
Dust cocoa powder over the set panna cotta if desired, then serve cold.