Heat oven to 350°F. To make red velvet cake for Connie’s trifle dessert, you can use a greased 9×13 pan. Otherwise, butter or spray two 9” round pans.
In a medium bowl, whisk together flour, salt, and cocoa; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, combine the sugar and oil, and beat on medium speed until well combined. Add eggs, one at a time, beating well after each addition. Add food dye and vanilla, and beat until well combined. Add flour mixture, alternating with buttermilk, scraping the sides of the bowl with a rubber spatula as needed.
In a small bowl, mix baking soda and vinegar until combined. Add to batter, and beat for 10 seconds. Evenly divide batter between the prepared pans. If making cupcakes, filling each cup about 2/3 of the way up. You may use an offset spatula to smooth down the batter into the liners.
Bake for 35-40 minutes until a toothpick comes out clean. Cool cake(s) in the pans for 10 minutes. Remove cakes from the pans, and transfer to a wire rack to cool completely.