For the cake: Preheat the oven to 350 degrees F. Grease and flour a 10-cup bundt pan, shaking out the excess flour.
Stir together the flour, sugar and salt in a large bowl; set aside. Whisk together the buttermilk, baking soda, vanilla and maple extracts and eggs in a separate bowl. Set aside.
In a medium saucepan over medium heat, melt the butter. Stir in the pumpkin puree and pumpkin pie spice until well combined. Next, add the boiling water and whisk until the mixture is smooth.
Pour the pumpkin mixture into the dry ingredients and whisk together until the flour is just mixed in. Slowly add the wet ingredients and stir until just combined. Pour the cake batter into the prepared bundt pan. Bake until a cake tester inserted into the center of the cake comes out clean, about 45 minutes.
For the rum syrup: Meanwhile, in a large saucepan over medium heat, combine the butter, sugar and 2 tablespoons water and bring to a boil; cook for 3 to 4 minutes. Turn off the heat. Slowly and carefully, pour in the rum while stirring. Then stir in the vanilla. Set aside.
Remove the cake from the oven and allow it to cool in the pan for 10 to 12 minutes. Carefully, invert the cake onto a rack over a baking sheet. Poke the top and sides of the cake all over with a wooden skewer.
Pour half of the syrup into the base of the bundt pan. Gently return the cake to the pan. Using the same skewer, poke the bottom of the cake. Pour the remaining syrup over the cake and around the edges of the pan. Allow the cake to cool completely and soak up the syrup in the bundt pan, about 1 hour.
Once cooled, invert onto a serving platter. Slice and serve each piece with a dollop of Maple Whipped Cream and a sprinkle of pumpkin pie spice.
Maple Whipped Cream:
Add the cream and maple syrup to the bowl of a stand mixer fitted with a whisk attachment. Whip on medium-high speed until soft peaks form, 2 to 3 minutes. Refrigerate until ready to use.