For the cake: Preheat the oven to 375 degrees F. Butter three 8-inch round cake pans, line with parchment and butter the parchment; set aside. Whisk the flour, cocoa powder, baking powder, baking soda and salt in a medium bowl; set aside. Whisk the buttermilk, food coloring and vanilla in a spouted measuring cup; set aside.
Beat the granulated sugar and butter in a stand mixer (or a large bowl if using a hand mixer) on medium speed until very light in color and fluffy, about 5 minutes, scraping down the sides of the bowl as needed. With the mixer still running, slowly add the eggs and beat until fully incorporated. Reduce the mixer speed to the lowest speed; with it running, add 1/3 of the flour mixture, then 1/2 of the buttermilk mixture, then 1/2 of the remaining flour mixture, then the remaining buttermilk mixture, then the remaining flour mixture. Scrape down the sides and beat until well mixed. Divide the batter evenly among the three prepared pans. Bake until slightly puffed and a toothpick comes out clean when inserted in the center, 20 to 25 minutes. Let cool 15 minutes in the pan, then turn out to cool completely.
For the frosting: Put the flour in a small saucepan. Vigorously whisk in about 1/2 cup of the milk, making sure to get the whisk into the edges of the pan, until you have a smooth, thick paste. (This step keeps the flour from clumping.) Slowly whisk in the remaining milk until fully incorporated and the mixture is smooth. Cook over medium heat, whisking continuously, until the mixture is very thick, about 5 minutes. (Toward the end of the process, the mixture will become a thick paste; it may seem to be forming lumps, but whisk vigorously and the lumps will disappear.) Scrape the frosting into a bowl, press plastic wrap onto the surface and refrigerate until cool, about 45 minutes.
Using a stand mixer fitted with the paddle attachment (or a hand mixer), beat the butter and granulated sugar on medium speed until very light and fluffy, about 5 minutes. Add the cooled flour mixture a tablespoon at a time and beat until smooth. Switch to the whisk attachment (or continue with the hand mixer), add the vanilla, salt and 3 to 5 drops food coloring if using and whip until very light and fluffy, like thick whipped cream, up to 10 minutes with a stand mixer or 12 minutes with a hand mixer.
To assemble: Place one layer on a cake plate, spread frosting on top and repeat with the 2 remaining layers. Frost the sides and decorate with candies and sanding sugar if using.
This recipe can instead be baked in two 9-inch layers or as 24 cupcakes.
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