Add heavy cream, whole milk and dark brown sugar to a saucepan. Bring to a simmer and then turn off the heat.
Whisk together the egg yolks, pumpkin pie spice and vanilla in a small bowl, about three minutes of whisking.
To gently heat the eggs, we ladle in a scoop of the warm cream mixture and whisk. We repeat this step a few times.
We add the yolk mixture back to the remaining cream mixture in the saucepan and stir in the pumpkin purée.
From here, we cook the pumpkin ice cream custard base until it thickens enough to cover a wooden spoon.
To prevent a film from forming, we directly cover the surface of the custard with plastic wrap and refrigerate until the mixture chills.
Now it's time to churn! In the last few minutes of churning, we add the bourbon. We pour the mixture in an airtight container and freeze for several hours.