1. Preparing the Blue Velvet Cake Batter

1. Preheat the oven to 350°F (175°C) and grease two 9-inch cake pans.
2. In a large bowl, combine all-purpose flour, granulated sugar, baking soda, and salt.
3. In another bowl, whisk together buttermilk, vegetable oil, eggs, cocoa powder, blue food coloring, vanilla extract, and white vinegar.
4. Gradually add the wet ingredients to the dry ingredients and mix until smooth.

2. Baking the Cake Layers

1. Pour the batter evenly into the prepared cake pans.
2. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
3. Allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

3. Making the Cream Cheese Frosting

1. In a large bowl, beat together unsalted butter and cream cheese until smooth and creamy.
2. Gradually add powdered sugar, 1 cup at a time, and continue to beat until smooth and fluffy.
3. Mix in vanilla extract until well combined.

4. Assembling and Decorating the Cake

1. Place one cake layer on a serving plate.
2. Spread a generous layer of cream cheese frosting on top.
3. Place the second cake layer on top, and apply a crumb coat (thin layer of frosting) around the entire cake.
4. Chill the cake for 15 minutes to set the crumb coat.
5. Frost the cake with the remaining cream cheese frosting.