Preheat oven to 350F. Grease a 15x10-inch rimmed baking pan. Set aside.
Stir flour, sugar, baking soda, and salt together in a medium bowl. Set aside.
In a heavy saucepan over low heat, combine butter, water, and cocoa powder. Bring to boil, stirring constantly.
Remove from heat, and stir in eggs, buttermilk, vanilla, and food color.Stir until well-combined.
Stir in flour mixture until combined.
Pour batter into prepared pan.
Bake for 20-30 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan to room temperature.
To make the frosting, beat cream cheese, butter, and vanilla together until well-combined.
Stir in powdered sugar and enough milk to make desired consistency.
Frost cooled bars with cream cheese frosting.
Chill any leftovers.
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Butter: I use salted butter in this recipe. If you use unsalted butter instead, increase the salt in the cake to 1/2 teaspoon and add a pinch or two of salt to the frosting.
Cocoa powder: I use regular unsweetened cocoa powder. I haven't tried Dutch-processed cocoa powder in this recipe.
Buttermilk: Regular or low-fat is ok. If you don't have buttermilk, you can make an easy substitute with this recipe.