Preheat oven to 325 degrees.
Step 1: Butter three cake pans, line with parchment and butter the parchment. Dust with flour, and knock out and discard excess flour.
Step 2: In a medium bowl, whisk together cocoa powder, food coloring, and 1/4 cup boiling water. Set aside to cool.
Step 3: In the bowl of an electric mixer fitted with the paddle attachment, cream together butter and shortening on high speed until smooth. Add sugar and continue beating until light and fluffy, about 5 minutes. Add eggs, one at a time, beating well after each addition.
Step 4: Stir buttermilk and vanilla into cool cocoa mixture.
Step 5: In another medium bowl, sift together cake flour and salt. With the mixer on low, add flour mixture alternating with cocoa mixture in three separate additions, beginning and ending with flour mixture. Beat until incorporated.
Step 6: In a small bowl, mix together vinegar and baking soda until baking soda dissolves; mixture will fizz. Add to batter and mix until just combined.
Step 7: Divide the batter between all three pans. Use your spatula to spread the batter evenly. Bake the cakes until a toothpick inserted in the center of the cakes comes out clean, about 30 minutes (rotate the pans half-way through). Transfer the cakes to a wire rack and cool for 20 minutes. Invert the cakes onto the rack and let them cool completely. Remove the parchment.
Step 1: Whisk together sugar and flour in medium heavy-bottomed saucepan. Add milk and cream and place over medium heat, whisking occasionally until mixture comes to a boil and has thickened, about 20 minutes.
Step 2: Transfer mixture to the bowl of an electric mixer fitted with the paddle attachment. Mix on high speed until cool. Reduce speed to low and add butter; mix until well incorporated. Increase speed to medium-high and mix until frosting is light and fluffy.
Step 3: Add vanilla and cinnamon and continue mixing until combined. If frosting is too soft, chill slightly and then re-mix to proper consistency.
Step 4: If frosting becomes too firm, place bowl over a pot of simmering water and re-mix to proper consistency.
Place one cake layer on serving platter. Trim the cake layer to create a flat surface and evenly spread approximately 1 1/4 cups of the frosting on the layer. Add the next two layers to this one and repeat. Crumb coat the cake and refrigerate briefly (approx 15 minutes). Frost the sides and top of cake with the remaining frosting. Garnish the cake with red hots and refrigerate for 15 minutes to firm up the entire cake.
This cake will keep beautifully in a cake saver at room temperature (cool and humidity free) for up to 3 days. If your room is not cool, place the cake in a cake saver and refrigerate for up to 3 days. Remove the cake from the refrigerator for at least 2 hours before serving,