Preheat the oven to 350 degrees F. Grease and flour two 9-inch round cake pans. Place a piece of parchment paper that’s been trimmed to fit in the bottom of each cake pan.
MAKE THE RED VELVET CAKE:
In a medium bowl, sift together the flour, cocoa powder, baking soda, and salt through a fine mesh sieve, using a spoon to push it through.
In a large bowl, use an electric mixer to combine the butter, sugar, and brown sugar until it looks sandy. Beat in the eggs, and then the buttermilk, vanilla, distilled white vinegar, and oil.
Beat in half of the dry ingredients until incorporated, and then beat in the rest. Add gel coloring and beat that in until evenly distributed.
Divide the batter between the two pans. Bake 35 to 40 minutes, or until a toothpick inserted into the middle of the cake comes out with just a few crumbs attached. Let the cakes cool for 10 minutes in the pans, then run a knife along the edge of the pans and turn out the cakes onto cooling racks to cool completely.
PREPARE THE FROSTING:
In a large bowl, use an electric mixer to combine the cream cheese and butter. Mix in the vanilla and salt. Add half of the powdered sugar, and mix until incorporated. Add the rest of the powdered sugar and mix again. Beat until the frosting is thick and creamy. Add more powdered sugar if you want it thicker. Thin it out with a little milk if you want it thinner.
Place one of the cake layers on a serving platter. Top it with frosting and spread to cover. Place the 2nd cake layer on top. Add the rest of the frosting to the top and the sides of the cake. Refrigerate cake until ready to serve. Keep leftovers stored in fridge.