Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the sifted flour, sugar, cocoa powder, salt, and baking soda using a whisk until well blended.
2 cups all-purpose flour, 2 cups granulated sugar, 1 cup unsweetened cocoa powder, 1 teaspoon baking soda, 1/2 teaspoon salt
In a separate saucepan, melt the butter over medium heat. Once melted, add the vegetable oil, buttermilk, eggs, vanilla extract, and boiling water. Stir until smooth and the mixture is warm but not boiling.
1 cup vegetable oil, 1 cup buttermilk, 2 large eggs, 1 teaspoon vanilla extract, 1 cup boiling water, 1/2 cup unsalted butter
Gradually pour the wet mixture into the dry ingredients, stirring continuously with a whisk or spatula until the batter is smooth and homogeneous. The batter will be thin.
2 cups all-purpose flour, 2 cups granulated sugar, 1 cup unsweetened cocoa powder, 1 teaspoon baking soda, 1/2 teaspoon salt, 1 cup vegetable oil, 1 cup buttermilk, 2 large eggs, 1 teaspoon vanilla extract, 1 cup boiling water, 1/2 cup unsalted butter
Pour the batter into a greased 9x13 inch baking sheet, spreading evenly with a spatula. Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted in the center comes out clean. The cake should spring back when gently pressed.
While the cake bakes, prepare the frosting by melting the butter in a saucepan over low heat. Remove from heat and stir in powdered sugar, cocoa powder, milk, and vanilla until smooth and glossy. If it's too thick, add a little more milk to reach spreading consistency.
2 cups powdered sugar, 1/3 cup unsweetened cocoa powder, 4 tablespoons milk, 1 teaspoon vanilla extract
Once the cake has cooled slightly, spoon the warm frosting over the top, spreading evenly with a spatula. Allow the frosting to set for about 15 minutes before slicing and serving.
