Preheat the oven to 350°F (180°C) or 160°C fan oven. Grease or line a 9x13 inch pan (23x33 cm) with parchment paper and set aside.
In a large mixing bowl, whisk together the sugar, vegetable oil, pumpkin puree, baking soda with water, vanilla extract, cinnamon or pumpkin pie spice, and salt until well combined.
Add the eggs to the mixture, whisking until well incorporated.
Add the flour to the mixture, whisking just until combined.
Pour the batter into the prepared pan, and gently tap it a couple of times on the counter to remove any air bubbles. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely.
While the cake cools, prepare the frosting. Sift powdered sugar (I like to do this over a piece of parchment paper, so it’s easier to add to the bowl without any mess)
In a separate large bowl, cream together the butter and cream cheese on medium speed with an electric hand mixer until well blended, about 2 minutes. Add in the vanilla extract and salt, mixing again.
Gradually add the powdered sugar, one cup at a time, mixing on low speed until all the powdered sugar is incorporated. Beat the frosting until smooth and creamy.
Once the cake is cooled, evenly spread the frosting on top. Decorate with pumpkin candies, candy corn, or as desired.