Preheat oven to 350 degrees. Line standard muffin tins with paper liners. Whisk together cake flour, cocoa, and salt.
With an electric mixer on medium-high speed, whisk together sugar and oil until combined. Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed. Mix in food color and vanilla.
Reduce speed to low. Add flour mixture in three batches, alternating with two additions of buttermilk, and whisking well after each. Stir together the baking soda and vinegar in a small bowl (it will foam); add mixture to the batter, and mix on medium speed 10 seconds.
Divide batter evenly among lined cups, filling each three-quarters full. Bake, rotating tins halfway through, until a cake tester inserted in centers comes out clean, about 20 minutes. Transfer tins to wire racks to cool completely before removing cupcakes. Cupcakes can be stored overnight at room temperature, or frozen up to 2 months, in airtight containers.
To finish, use a small offset spatula to spread cupcakes with frosting.
Gel-paste food color is much more concentrated than the supermarket liquid variety; if you substitute the liquid, you may need to add an entire bottle (1.5 ounces) to achieve the desired shade.