In a large bowl, combine all dry ingredients. Mix in the oil, buttermilk, and eggs. Once the eggs are fully incorporated, pour in the hot water and mix well (your batter will seem runny, but I promise that it will work out!).
Grease a 9×13″ baking pan and pour batter into pan. Bake for 33-38 minutes (or until a toothpick inserted into the middle of the cake comes out clean- mine was done at 33 minutes). When the cake is completely cool, top with frosting and decorate with sprinkles (if so desired).
Mix all ingredients with a hand mixer until smooth. Add more powdered sugar or milk until you reach desired consistency.