In a small bowl, sprinkle the gelatin over the water and let soak about 10 minutes (do not stir). Meanwhile, in a medium saucepan, heat the milk, vanilla bean, ground coffee, and sugar to a simmer over medium heat, stirring occasionally to dissolve the sugar. As soon as it simmers, turn off the heat and add the gelatin mixture, stirring to dissolve the gelatin. If the gelatin doesn't completely dissolve in 3 minutes, return the mixture to the heat and warm gently until dissolved. Strain through a fine strainer or chinois. Pour the mixture into 6 to 8 ramekins or dessert cups. Chill, uncovered for 2 hours. Dip the cups in hot water for 10 seconds, then turn the panna cottas out onto dessert plates (or, serve in the cups).