Preheat oven to 350 degrees F (175 degrees C). Grease and flour three 9 inch round cake pans.
In mixer bowl combine flour, sugar, cocoa, baking powder and salt. Gently stir together and make a divot in the dry ingredients. Add eggs, coffee, buttermilk, oil and vanilla. Beat for 2 minutes on speed 2 (or medium speed). Fold in diced maraschino cherries. Batter will be very thin. Pour into prepared pans.
Bake at 350 degrees F (175 degrees C) for 30 to 40 minutes, or until toothpick inserted into center of cake comes out clean. Cool for 10 minutes, then remove from pans and finish cooling on a wire rack. Put in freezer once cool for at least 1 hour.
Frosting: Whisk milk into flour and cornstarch in small saucepan until smooth. Heat and stir until it boils and thickens. Cool thoroughly (at least 2 hours in fridge)
Cream butter, sugar and vanilla in bowl for 8 minutes. Add 1/2 of the thickened milk and mix on medium speed for 2 minutes. Add remainder of thickened milk and mix on medium speed for 5 minutes. Mixture should resemble whipped cream.
Remove 1 cake layer from freezer and frost top and leave the edges higher than the middle with the frosting. Layer 1/3 of the cherry pie filling on frosting. Cover with next layer and repeat frosting and pie filling. Layer final cake layer and frost entire cake. Put chocolate sprinkles on the sides of the cake. Using a piping bag and decorator tip, pipe around the bottom of the cake and along the top. Spread remaining 1/3 of the cherry pie filling on the top of the cake. Refrigerate overnight before serving.