Make the custard by combining 2 cups cream, the cinnamon, nutmeg, and ginger in a saucepan over medium-high heat. Halve the vanilla bean lengthwise; scrape out the seeds with a knife, then add the seeds and pod to the saucepan. Bring to a boil and remove from heat. Whisk the egg yolks with the granulated sugar in a bowl until dissolved. Add the remaining 1-1/2 cups cream and the milk to the saucepan. Slowly whisk in the yolk mixture, then strain the custard through the sieve back into the bowl. Whisk in the pumpkin.