Preheat the oven to 350 degrees.
Sift the flour, baking soda, salt, cinnamon, and pumpkin spice in a bowl and set aside.
With an electric mixer, combine room temperature butter and sugar for about 5-minutes at high speed until the texture is light and fluffy.
Mix in the eggs, vanilla, and pumpkin purée. Combine for 1-minute at a medium-low speed.
Mix in the dry ingredients. Combine for about 30-seconds.
Transfer batter to a greased baking dish (I used a ceramic 6 x 10 pan) and bake for about 38-45 minutes. Baking time will depend on the size and material of your pan. Pay attention to the bars while they are cooking, and use the toothpick method to determine whether they're done. The edges will be golden brown, and the toothpick should come out clean after poking the center. Let the bars cool for about 40-minutes on the countertop or 25-minutes in the refrigerator. You'll want them slightly warm while applying the frosting.