• Heat the oven to 350 F.
• Lightly spray 2 9-inch round cake pans with cooking spray and line them with cut-out rounds of parchment paper. Set aside.
• In a medium bowl, sift together the flour, cocoa powder, baking soda, and salt; set aside.
• Combine the butter and sugar in a large mixing bowl with an electric mixer (stand mixer if possible); beat until light and fluffy.
• Add the eggs one at a time, beating thoroughly after each addition.
• Beat in the vegetable oil, vanilla, and food coloring.
• With the mixer on medium speed, beat in the buttermilk and dry ingredients until blended.
• Add the vinegar and beat on low speed until blended.
• Divide the mixture between the two prepared cake pans.
  
• Bake in the preheated oven until a toothpick inserted in the center comes out with a few crumbs clinging
• Cool in the pans for 10 minutes, then remove to a rack to cool completely.
  

Prepare the Frosting

• Place a mesh strainer over a bowl and set aside.
• Combine the milk and flour in a saucepan or saucier and whisk until smooth. Place the pan over medium heat and cook while whisking constantly until it begins to thicken, about 2 minutes.
• Add the sugar and whisk to blend. Continue cooking and whisking until the mixture is bubbling and pudding-like in thickness, about 5 to 7 minutes longer.
• Pour the hot mixture through the strainer into the bowl and whisk in the vanilla.
• Place a sheet of plastic wrap directly on the surface of the hot mixture to prevent a skin from forming.
  
• Let it cool completely. Place the bowl in a larger bowl with some ice water for quicker cooling.
• Beat the butter until light and fluffy, then beat in the cooled flour mixture a little at a time.
• Switch to the whisk attachment and continue beating for 3 to 4 minutes until light and fluffy.
  
• Frost the cake.