Bring the ingredients to room temperature. Pull out any refrigerated items 30-40 minutes before baking. Preheat the oven to 350° F. Grab a 12-cup bundt pan and a large baking sheet. Set them aside.
Whisk the dry ingredients. In a large bowl, whisk together the gluten-free flour, pumpkin pie spice, cinnamon, baking powder, baking soda, salt, and any mix-ins if using.
Mix the wet ingredients. In the bowl of a stand mixer with a whisk attachment, on medium speed, beat together the pumpkin, sour cream, melted coconut oil, granulated sugar, dark brown sugar, and vanilla extract until smooth. Add the room-temperature eggs one at a time until combined.
Combine the wet and dry ingredients. On low speed add the dry ingredients. Once the dry ingredients are added, turn up the speed to medium and continue to mix until the flour is fully incorporated.
Add the cake batter to the greased bundt pan. Using a rubber spatula, scrape down the sides of the bowl and fold in any remaining flour bits. Generously grease the 12-cup capacity bundt pan with additional melted coconut oil. Pour the batter into the prepared bundt pan and smooth the top. Tap the pan a few times to remove any air bubbles. Bake. Place the bundt cake pan on top of the baking sheet and into the oven to bake for 60-70 minutes. The bundt cake is ready when a toothpick inserted in the center of the cake comes out clean with a few moist crumbs. If using a digital thermometer, the internal temperature should be at least 200F.
Let the cake cool. Carefully remove the bundt pan from the oven. Allow the bundt cake to cool in the pan for 10-15 minutes. Then carefully invert the cake onto a wire rack to finish cooling. The cake needs to be completely cooled before frosting.
Make the frosting. Add the softened butter and cream cheese to a stand mixer with a whisk or paddle attachment. Beat together on high speed until smooth and creamy. Scrape down the sides of the bowl. Add the powdered sugar, vanilla extract, and salt. Beat on low speed for 30 seconds and then switch to high speed and beat for 1-2 minutes or until light fluffy and fully combined.
Frost the cake. Using a piping bag with a 1M piping tip, decorate the cooled cake as desired.