Sift dry ingredients and spices together in a medium bowl. Flour, baking powder, cinnamon and salt.
Grease, or line a 13" x 9" baking pan.
In a stand mixer or with a hand mixer at medium speed, beat butter, brown sugar, and vanilla until blended.
Add eggs, one at a time to sugar mixture; beat well after each addition. Add pumpkin, and mix until incorporated.
Fold dry ingredients into wet ingredients, just until blended. Spread batter into a 13" x 9" pan. Smooth surface to make sure it's even.
Bake at 350 degrees F. for 25 minutes, or until a toothpick inserted into the middle comes out clean.
Cool completely on a wire rack before frosting.