Generously coat bundt pan with cooking spray, dust with flour and tap out the excess flour. Position rack in lower third of oven. Preheat oven to 350º.
In medium bowl, whisk together the sifted flour, baking powder, baking soda, salt and spices. Set aside.
Add granulated sugar to large bowl. Crumble in dark brown sugar, breaking up any clumps with your fingers. Add oil and 2 eggs and beat until blended then beat in the remaining eggs and the pumpkin puree. Scrape down the bowl. Alternately add the buttermilk and dry ingredients. When everything is blended, scrape down the bowl, then beat a few seconds longer until the batter is smooth and creamy.
Scoop the batter into the prepared pan.
Bake for 65-70 minutes or until cake has risen, is springy to the touch and is cracked on top, and a knife inserted in the center comes out clean. Cool the cake in its pan on a rack for 20 minutes. Slide a knife around the pan sides to release the cake. Top the cake pan with a serving platter, invert, and lift off the pan. Cool the cake completely.
Whisk together all the ingredients until smooth and creamy. Add a drop more of milk if necessary. The glaze should have a soft, just slightly runny (but not liquefied) consistency. Drizzle the glaze on the top center of the cake, letting it drip down the sides.