Preheat the oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan or spray with nonstick spray and set aside.
In a large bowl, beat 8 tablespoons room-temperature butter and 1 1/2 cups sugar with a hand or stand mixer until light and fluffy, about 2–3 minutes.
Add 1/4 cup red liquid food coloring (or 1 tsp gel), 2 teaspoons vanilla extract, then add the 2 eggs one at a time, mixing just until incorporated after each addition.
Whisk together 2 1/4 cups all-purpose flour, 5 tablespoons unsweetened cocoa powder, 1 teaspoon salt, and 1 teaspoon baking soda in a separate bowl.
Alternate adding the dry ingredients and 1 cup buttermilk combined with 1 tablespoon distilled white vinegar to the butter mixture, mixing by hand with a wooden spoon or on low speed just until combined; do not overmix.
Pour the batter into the prepared pan and spread it into an even layer. Bake at 350°F for 25–30 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and use a fork to poke holes all over the warm cake. Pour the 12 oz sweetened condensed milk evenly over the cake (use whole can or about half, as desired). Allow the cake to cool completely.
For the frosting, beat 1/2 cup room-temperature butter and 8 oz room-temperature cream cheese on high until smooth and fluffy, about 2–3 minutes.
Reduce mixer speed and add 3 cups powdered sugar, 1/4 teaspoon salt, 1 1/2 teaspoons vanilla extract, and 2 tablespoons milk or heavy cream. Beat an additional 2–3 minutes until the frosting is light and spreadable.
Spread the frosting evenly over the cooled cake and decorate as desired.