Adjust an oven rack to the middle position and heat to 350 degrees. Grease, line with parchment or wax paper, and dust with cocoa powder two 9-inch cake pans.
Whisk the flour, baking soda, and salt together in a medium bowl. In another medium bowl, whisk the buttermilk, eggs, vinegar, and vanilla extract together. In a small bowl, mix 2 tablespoons of the cocoa and red food coloring together to a smooth paste.
In a large bowl, beat the butter and sugar together with an electric mixer on medium-high speed until light and fluffy, 3 to 6 minutes. Reduce the mixer speed to low and beat in one-third of the flour mixture, followed by half of the buttermilk mixture. Repeat with half of the remaining flour mixture and the remaining buttermilk mixture. Beat in the remaining flour mixture until just combined. Beat in the cocoa mixture until the batter is uniform.
Give the batter a final stir with a rubber spatula to make sure it is thoroughly combined. Scrape the bater into the prepared pans, smooth the tops, and gently tap the pans on the counter to settle the batter. Bake the cakes until a toothpick inserted in the center comes out with a few crumbs attached, about 25 minutes, rotating the pans halfway through baking (I didn't do this).
Let the cakes cool in the pans for 10 minutes. Run a small knife around the edge of the cakes, then flip them out onto a wire rack. Peel off the parchment paper, flip the cakes right side up, and let cool completely before frosting, about 2 hours.
Line the edges of a cake platter with stripes of parchment or wax paper to keep the platter clean while you assemble the cake. Place one of the cake layers on the platter (before doing this, place a small blob of icing in the center to hold the cake in place). Spread 1 cup of the frosting over the top, right to the edge of the cake. Place the other cake layer on top and press gently to adhere. Frost the cake with the remaining frosting (I did a crumb coat first, then sealed the cake with more icing) and remove the parchment strips from the platter before serving.
***Cream Cheese Frosting***.
Beat the cream cheese, butter, sour cream, vanilla, and salt together in a large bowl with an electric mixer on medium-high speed until smooth, 2-4 minutes.
Reduce mixer speed to medium-low, slowly add the confectioners' sugar, and beat until smooth, 4 to 6 minutes. Increase the mixer speed to medium-high and beat until the frosting is light and fluffy, 4 to 6 minutes.