• 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.
  Cooking spray for the pans
• In a medium bowl, sift together the cake flour, cocoa powder, baking soda, and salt; set aside.
  2 3/4 cups cake flour | 1 tsp baking soda | 1/2 tsp fine salt | 1/4 cup cocoa powder
• Combine the butter and sugar in a large mixing bowl with an electric mixer; beat until light and fluffy.
  1/2 cup unsalted butter | 1 1/2 cups granulated sugar
• Add the eggs one at a time, beating thoroughly after each addition.
  3 large eggs
• Beat in the vegetable oil, vanilla, and food coloring.
  3/4 cup vegetable oil | 2 tsp vanilla extract | 1 tbsp red gel food coloring or 2 tablespoons liquid
• With the mixer on medium-low speed, add the buttermilk and dry ingredients and beat until blended.
  1 cup buttermilk
• Add the vinegar and beat on low speed until blended.
  1 tbsp white vinegar
• 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 the layers to a rack to cool completely.
  

Prepare the Frosting

• Place a mesh strainer over a bowl and set aside.
• Combine the milk and 1/2 cup of flour in a saucepan and whisk until smooth. Place the pan over medium heat and cook, whisking constantly, until it begins to thicken, about 2 minutes.
  1 1/2 cups milk | 1/2 cup all-purpose flour
• Add the 1 1/2 cups of granulated sugar and 1/4 teaspoon of salt, and whisk to blend. Continue cooking and whisking until the mixture is bubbling and pudding-like in thickness, about 5 to 7 minutes longer.
  1 1/2 cups granulated sugar | 1/4 tsp fine salt
• Pour the hot mixture through the strainer into the bowl and whisk in the vanilla.
  1 tbsp vanilla extract
Straining the mixture removes any lumps that could result in a grainy frosting. A smooth base is the key to light, silky ermine frosting.
• Place a sheet of plastic wrap directly on the surface of the hot mixture to prevent a skin from forming.
  
• Let it cool completely. For quicker cooling, place the bowl in a larger bowl of ice.
The flour mixture must be completely cool before beating in the butter. If it's too warm, the butter will melt and the frosting will be soupy.
• Beat the butter until light and fluffy, then beat in the cooled flour mixture a little at a time.
  1 1//2 cups unsalted butter
• Switch to the whisk attachment and continue beating for 3 to 4 minutes, or until the frosting is light and fluffy.
  
• Assemble the cake. Place a cake layer on a plate and spread with some of the ermine frosting. Top with the second layer and frost the top and sides of the cake.