Pre-heat oven to 350°F. Generously brush the inside of a Bundt® pan with very soft butter, sprinkle with flour, and tap out the excess. Set aside.
Sift together the flour, baking powder, baking soda, salt, and spices. Set aside.
In the bowl of an electric mixer, beat the butter on medium-low speed with the paddle attachment until smooth. Add in both sugars and mix on medium until light and fluffy, 3 to 5 minutes. Add the oil and vanilla. Mix until combined.
With the mixer on low, add in the eggs, one at a time. Stop the mixer and scrape down the sides and bottom of the bowl.
With the mixer on low, add in half of the flour mixture and mix until just combined. Add in the buttermilk and mix until combined. Add in the remaining half of the flour mixture and mix until just incorporated and few streaks of flour remain visible. Add in the pumpkin puree and mix until smooth. The batter will be thick.
Spoon the cake batter into the prepared pan. Tap the bottom of the filled pan on a hard work surface to release any air bubbles. Bake for 45 to 55 minutes, or until a long wooden skewer or thin paring knife inserted into the center of the cake comes out clean. Cool on a wire rack for about 15 minutes, or until the cake can safely be handled.
Gently loosen the edges of the cake with a flexible rubber spatula (but resist the urge to run a knife around the edges or it may cut into the cake). Place a wire rack or cutting board on top of the cake. Holding on to the edges of the pan and rack, carefully invert everything. The cake pan will still be warm, so use oven mitts as needed. Remove the cake pan and continue to completely cool the cake before adding the glaze.