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

Prepare the Frosting

1. Place a mesh strainer over a bowl and set aside.
2. 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.
3. 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.
4. Pour the hot mixture through the strainer into the bowl and whisk in the vanilla.
5. Place a sheet of plastic wrap directly on the surface of the hot mixture to prevent a skin from forming.
6. Let it cool completely. Place the bowl in a larger bowl with some ice water for quicker cooling.
7. Beat the butter until light and fluffy, then beat in the cooled flour mixture a little at a time.
8. Switch to the whisk attachment and continue beating for 3 to 4 minutes until light and fluffy.
9. Frost the cake.