Preheat the oven to 350 degrees F. Line a 12-cup muffin tin with paper liners.
Sift the flour, sugar, cocoa, baking soda and salt into a large bowl. Whisk together the buttermilk, oil, sour cream, food coloring, vinegar, vanilla and egg together in medium bowl.
Add the buttermilk mixture to the flour mixture and stir until just combined. Divide evenly among the cupcake liners. Bake until a toothpick inserted in the center of a cupcake comes out with a few moist crumbs, 18 to 20 minutes. Remove the cupcakes from the tin and cool completely on a wire rack.
For the frosting:
Bring a few inches of water to a boil in a saucepan that can hold a bowl comfortably without touching the water. Whisk the sugar, lemon juice, cream of tartar, salt and egg whites in the bowl. Set the bowl above the boiling water and continue whisking until the mixture is hot to the touch and the sugar dissolves, about 2 minutes. Remove the bowl from the water and whip with an electric mixer on medium-high speed until the meringue is cool and holds a stiff peak, about 7 minutes.
Spoon or pipe the frosting decoratively and generously on the cupcakes.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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