Pre-heat oven to 350F. Grease 2 9-inch baking pans then put greased parchment paper at the bottom for easy release.
In a medium sized bowl, sift 2 1/4 cups cake flour, 1 teaspoon salt and 2 tablespoons cocoa powder. Set aside.
Using an electric mixer or a stand mixer with a paddle attachment, cream 1/2 cup butter and 1 1/2 cups sugar on medium speed until light and fluffy (about 2 minutes).
Add the eggs one at a time until combined.
In a separate bowl, add your red food colour to 1 cup buttermilk and stir until combined.
Switch to low speed and add a third of your flour mixture into the batter, then half of your buttermilk, then another third of the flour, the last of the buttermilk, then finally putting in the rest of the flour making sure each addition is fully combined before adding in the next. Scrape the bottom and sides of your bowl as needed.
Add 1 teaspoon vanilla and stir until you see no more streaks of flour in the batter. Remove the bowl from stand mixer, if using.
In a small bowl, mix 1 teaspoon baking soda with 1 teaspoon vinegar. Once the mixture foams and bubbles, quickly fold it into your cake batter.
Pour the batter into your prepared baking pans and bake for 24-30 minutes or until the cakes start to pull away from the sides of the pans and when a toothpick inserted in the middle comes out clean.
Take out from oven and cool on a rack for about 15 minutes or until the pans are cool enough to handle. Then remove the cakes from pans, remove the parchment paper from the cakes and let cool completely on the racks before frosting.
In a medium sized sauce pan, combine 5 tablespoons flour and 1 cup milk and constantly stir over medium heat until very thick, like pudding (8-10 minutes).
Remove from heat, place plastic wrap on top and cool to room temperature. Then transfer it to the fridge and chill for 1 hour.
Using an electric mixer or a stand mixer fitted with the whisk attachment, cream 1 cup sugar, 1 cup butter and 1 teaspoon vanilla extract on medium high speed until light and fluffy (3-5 minutes).
Add the completely cool flour mixture and continue beating until you get the consistency of whipped cream (3-5 minutes).
Fill, frost and decorate cake as desired.
While you can serve this cake immediately, it is at its best after at least a couple of hours in the fridge. Then let stand in room temperature for about 10 minutes before serving.