Preheat the oven to 325 degrees F. Spray a 9x13-inch rectangular baking pan with nonstick spray. Set aside. (NOTE: I use a glass baking dish for this recipe and keep the cake in the pan for the soak and when I serve it. If you want to remove the cake before adding the toppings, line the baking dish with parchment paper. Also, if you use a metal baking dish, reduce the bake time by 3 to 5 minutes). In a stand mixer fitted with the paddle attachment, combine the butter, granulated sugar, and brown sugar. Beat the ingredients on medium-high speed for about 3 to 5 minutes, or until the mixture is light in color and slightly increased in volume. Stir in the red velvet emulsion.
Scrape down the sides and bottom of the bowl and mix again for about a minute. Lower the speed and add the eggs, one at a time, mixing until each egg is incorporated before adding the next. Once all of the eggs are added, turn the mixer to medium-high and beat for another minute or two, until the mixture is smooth.
In a separate bowl, whisk together the flour, cocoa powder and salt.
With the stand mixer on low speed, alternately add the dry ingredients and the buttermilk into the butter mixture, beginning and ending with dry ingredients. Mix until just incorporated. Fold in the sour cream.
In a small bowl, combine the vinegar and baking soda until it fizzes and then add to the batter. Stir on low or fold in until blended.
Bake for about 30 to 35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs on it.
Let the cake cool completely before adding the soak.