In a small bowl, sprinkle the gelatin over ¼ cup water. Let stand until softened, about 5 minutes.
In a saucepan heat the half and half, sugar, cinnamon, nutmeg and pinch of salt until the sugar has dissolved and the mixture is hot but not boiling, about 3-5 minutes. Remove from heat, stir in gelatin mixture, buttermilk, pumpkin purée, vanilla and bourbon.
Strain mixture into a clean bowl or, ideally, a pourable Pyrex measuring cup. Pour mixture into martini glasses or ramekins and chill until set, 3 hours. I garnished mine with some candied pecans and store-bought hazelnut pirouette cookies, which is totally unnecessary, but complements the pumpkin flavor nicely. Also, if you don’t care about presentation, it is much easier to chill the panna cotta in ramekins: the martini glasses take up a lot of room in the refrigerator and are quite unstable.