In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg and salt. In the bowl of an electric mixer fitted with the flat beater, beat together the butter and brown sugar on medium-high speed until well combined. Beat in the eggs, one at a time, scraping down the sides of the bowl with a rubber spatula. Add the pumpkin puree and sour cream and mix. Stir in the flour mixture. The batter will be quite thick.