Adjust rack to lower third of oven; preheat oven to 350 degrees. Grease and flour a 10-inch angel food tube pan. For best results, have all ingredients at room temperature.
Sift together the flour, spices, baking powder, baking soda and salt; set aside. Using an electric mixer, preferably with a paddle attachment, beat the butter at medium speed until creamy and smooth. Add the brown and granulated sugars; beat until light. Add the eggs, 1 at a time, beating well after each addition. Reduce mixer speed to low; add the pumpkin puree and polenta. Add the flour mixture to the pumpkin mixture alternately with the milk, ending with the dry ingredients, and stir just until well blended. Pour the batter into the prepared pan, spreading it evenly.
Bake for 55 to 65 minutes, or until a cake tester comes out free of raw batter. Remove the pan from the oven and place on a wire rack. Let the cake cool in the pan for about 10 minutes. Invert the cake onto the wire rack; remove the cake pan. Let the cake cool completely before slicing.