Preheat the oven to 350 degrees F. Spray four 9-inch cake pans with nonstick cooking spray.
This cake batter will be made in two batches. You will color the first batch red and the second batch blue.
To make the first batter, sift the flour three times, then measure 2 1/2 cups. Combine the sifted flour with the cocoa and salt in a bowl. Set aside.
In another bowl, stir together the buttermilk and vanilla.
Using a stand mixer fitted with the paddle attachment, cream the sugar, shortening and eggs. With the mixer on low speed, add the flour mixture in three additions, alternating with the buttermilk mixture, beginning and ending with the flour mixture.
In a small bowl, stir together the baking soda, vinegar and red food coloring. Feast your eyes on the colorful chemical reaction! Fold the food coloring mixture into the cake batter. Pour the batter into two of the prepared cake pans.
Make the second batter in the same manner, using the blue food coloring. Pour the blue batter into the remaining two prepared pans.
Bake all four pans until a toothpick comes out clean, 15 to 20 minutes. Let the cakes cool in the pans on wire racks for 10 minutes. Remove the cakes from the pans and let cool completely wire racks.
Using a serrated knife, level the top of each cake layer. Place one layer on a serving plate; spread Boiled Icing over the top. Continue stacking and icing until you have used all of the layers. Cover the outside of the cake with Boiled Icing.
Cook the milk and flour in a saucepan over medium-high heat until it reaches the pudding stage (the consistency of glue), no longer than 10 minutes. Remove from the heat and let cool to lukewarm. Strain to remove any clumps of flour.
Using a stand mixer fitted with the paddle attachment, cream the sugar, butter, shortening, vanilla and salt on medium speed until nice and fluffy. One teaspoon at a time, add the pudding mixture. Beat on high speed for 2 to 3 minutes. Voila! Old-fashioned boiled icing.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.