Heat the raisins and rum in a spall saucepan over medium heat and bring to a boil. Reduce heat to low and simmer until the raisins have absorbed most of the liquid and have softened, about 6 to 8 minutes. Remove from heat and allow to cool to room temperature. Once cooled, mix with the brown sugar, cinnamon, and salt and set aside.

Preheat the oven to 350 degree Fahrenheit. In a medium-sized bowl, whisk together the flour, baking powder, cinnamon, salt, baking soda, and nutmeg. Set aside. In another small bowl, whisk together the pumpkin puree, buttermilk sour cream, and vanilla extract until smooth. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and granulated sugar at medium high speed until smooth, scraping down the sides of the bowl as-needed. Add the eggs one at a time, making sure the egg is incorporated before adding the next one. Reduce the speed to low, and alternate between adding the dry ingredients and the wet ingredients in three separate increments until they are all just combined. Add the rum and stir to incorporate.

Pour 1/3 of the batter into a 10-cup capacity bundt pan sprayed with cooking oil. Drag a spoon in the center of the ring to make a trench in the center of the batter that goes around the bundt. Sprinkle half of the rum raisin filling into the trench, then top with 1/3 of the batter and repeat the process with the remaining half of the rum raisin filling. Top with the remaining 1/3 of the batter. Bake in the oven until a toothpick inserted into the cake comes out relatively clean, about 60 minutes.