**Mix Dry Ingredients:** In a medium bowl, combine the flour, baking soda, baking powder, salt, and spices. Mix well and set aside.
**Mix Wet Ingredients:** In another bowl, whisk together the sugars and vegetable oil until combined. Add the eggs and vanilla, mixing well. Stir in the pumpkin puree until smooth.
**Combine Wet and Dry:** Gradually add the dry ingredients to the wet mixture, mixing until just combined.
**Bake the Cake:** Pour the batter into a greased 9×13-inch baking pan. Bake for about 30 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool completely before icing.
**Brown Butter Cream Cheese Icing:**
**Brown the Butter:** In a small saucepan, melt the butter over medium-high heat, stirring constantly until it turns golden brown. Quickly transfer the browned butter to a new container to cool and solidify (use the freezer to speed this up if needed).
**Prepare the Icing:** In a large bowl or the bowl of a stand mixer, beat the cooled browned butter and cream cheese on medium-high speed until fluffy, about 2 minutes.
**Add Flavorings:** Add the salt, cinnamon (if using), and vanilla, and mix until combined.
**Add Powdered Sugar:** Gradually add the powdered sugar, 1/2 cup at a time, mixing on low speed until fully incorporated. Increase the speed to medium-high and whip until the icing is light and fluffy, about 2 minutes.
**Ice the Cake:** Spread the icing evenly over the cooled sheet cake. This recipe makes a generous amount of icing, so use as much or as little as you prefer.
