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, granulated sugar, cocoa powder, baking soda, baking powder, and salt.
In a separate medium bowl, whisk together the buttermilk, vegetable oil, eggs, and 1 teaspoon vanilla extract until combined.
Add the wet ingredients to the dry ingredients and mix until just combined.
Carefully stir in the hot water until the batter is smooth and thin.
Pour the batter evenly into the prepared sheet pan.
Bake for 28 to 30 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 cools, prepare the cream cheese frosting: In a large bowl, beat the softened cream cheese and unsalted butter together until smooth and creamy.
Gradually add the powdered sugar, one cup at a time, beating well after each addition until the frosting is smooth and fluffy.
Beat in 1 teaspoon vanilla extract until fully incorporated.
Once the cake is completely cool, spread the cream cheese frosting evenly over the top.
Cut into 12 squares and serve.