Preheat the oven to 350˚F. Butter and flour a 10-inch bundt pan and set aside.
In a standing mixer fitted with a paddle attachment, or with a handheld electric mixer, cream together the butter and sugar until light and fluffy, about 2 – 3 minutes. Add the vanilla and beaten eggs and mix on medium speed until thoroughly combined.
In a separate bowl, stir together the pumpkin puree and the crème fraîche. In another bowl, stir together the flour, baking powder, salt, and spices.
With the mixer on low, add ⅓ of the dry ingredients followed by half of the wet ingredients, and mix until just combined. Alternate with the next ⅓ of the dry ingredients and the remaining half of the wet ingredients, ending with the last ⅓ of the dry ingredients. Continue to mix on low until everything is just combined and the batter is smooth.
Pour the batter into the prepared bundt pan and use a spatula to smooth the top. Bake in the preheated oven, turning once halfway through, until a toothpick inserted in the center comes out clean, about 50 minutes.
Meanwhile, to make the bourbon butterscotch sauce: In a medium saucepan, heat the butter, brown sugar, cream, and bourbon together over moderate heat. Stir with a wooden spoon until smooth. Once bubbling, leave the sauce without stirring for 3 minutes. Remove from the heat. Allow to cool until just warm before serving.
Once the cake has finished baking, remove it from the oven and place on a wire rack to cool for 5 minutes. Run a pairing knife around the inside edge of the cake, and invert the pan onto the wire rack to release the cake. Allow it to cool completely, or serve warm, drizzled with the bourbon butterscotch sauce.