Prep oven and pan. Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch pan or line the bottom with parchment paper.
Mix sugar and wet ingredients. In a large bowl, beat together the sugar, vegetable oil, pumpkin puree, and eggs until smooth and well blended.
Mix dry ingredients. In another bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, ginger, cloves, allspice, and nutmeg.
Combine. Gradually add the dry ingredients to the wet ingredients, mixing until just incorporated. Do not overmix.
Bake. Pour the batter into the prepared pan and spread evenly. Bake for 30-40 minutes. The cake will look very brown before it's actually done: don't trust your eyes! Test with a toothpick inserted into the center; it should come out clean or with just a few moist crumbs.
Cool. Let the cake cool completely in the pan before frosting. This will take awhile, be patient!
Make the frosting. In a medium bowl, beat together the melted butter and cream cheese until smooth. Gradually add the powdered sugar and vanilla, beating until the frosting is creamy and smooth. Start with 7-8 cups of powdered sugar and add more to taste.
Frost and serve. Spread the frosting evenly over the cooled cake. Cut into squares and serve right out of the pan.