Preheat the oven to 350°F (175°C). Grease and flour a 9×13-inch sheet pan.
In a large mixing bowl, sift together the flour, sugar, baking soda, baking powder, salt, and cocoa powder.
In a separate bowl, whisk together the vegetable oil, buttermilk, eggs, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry ingredients, mixing until smooth.
Slowly pour in the hot water and mix until the batter is thin and well combined.
Pour the batter evenly into the prepared sheet pan.
Bake for 28 to 32 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool completely in the pan on a wire rack.
While the cake is cooling, prepare the cream cheese frosting: In a medium bowl, beat the softened cream cheese and butter together until smooth and creamy.
Add the powdered sugar gradually, beating well after each addition until the frosting is smooth and fluffy.
Mix in the vanilla extract until fully incorporated.
Once the cake is completely cooled, spread the cream cheese frosting evenly over the top.
Slice into 12 squares and serve.