Preheat an oven to 350°F and place an oven rack in the middle position. Grease a 10-inch cast iron skillet with butter. Line the bottom with a round of parchment paper, then grease that with butter as well.
Place flour, pumpkin spice, baking soda and sea salt in a medium bowl, whisking to combine. Set aside. Place butter, white sugar and brown sugar in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium until light and fluffy (about 4 minutes).
Beat in eggs, 1 at a time, until incorporated. Add canned pumpkin, sour cream, bourbon and vanilla, mixing until well combined. At this point, it may look like the mixture has broken (lumpy), but that’s ok. In two additions, mix in dry ingredients just until combined. Stir in chocolate chips.
Transfer batter to the prepared skillet and smooth out the top using an offset spatula. Evenly sprinkle more chocolate chips, pepitas and turbinado sugar on top. I used about 2 tablespoons of chocolate chips, 4 tablespoons pepitas and 1 tablespoon turbinado sugar.
Bake until the center is completely set (35 to 40 minutes) and a toothpick inserted in the middle the cake comes out clean. Transfer skillet to a cooling rack and let cool completely before serving.
