Step 1: Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, or line with parchment paper. Set aside.
Step 2: In a medium bowl, whisk together the flour, baking soda, salt, and cocoa powder. Set aside.
Step 3: In a large mixing bowl, cream together the softened butter and granulated sugar with an electric mixer until light and fluffy, about 3-5 minutes.
Step 4: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract. Scrape down the sides of the bowl as needed.
Step 5: In a separate small bowl, combine the buttermilk, vegetable oil, and red gel food coloring. Whisk until the color is evenly distributed.
Step 6: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the buttermilk mixture, beginning and ending with the dry ingredients. Mix on low speed until just combined. Do not overmix.
Step 7: In a small cup, combine the white vinegar and an additional 1/2 teaspoon of baking soda. It will fizz immediately. Quickly fold this into the cake batter.
Step 8: Divide the batter evenly between the prepared cake pans.
Step 9: Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 10: Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely.
Step 11: To make the frosting: In a large bowl, beat the softened cream cheese and unsalted butter together until smooth and creamy, about 2-3 minutes.
Step 12: Gradually add the sifted powdered sugar, one cup at a time, beating until smooth after each addition. Add the vanilla extract and beat until the frosting is light and fluffy. If the frosting is too thick, add 1-2 tablespoons of milk or cream; if too thin, add more powdered sugar.
Step 13: Once the cakes are completely cool, frost your phantom red velvet cake generously. Decorate as desired.