Preheat oven to 350° F (177°C). Line muffin tins with 15 cupcake liners.
Combine cake flour, baking powder, salt, and cocoa together in medium mixing bowl. Sift mixture. Set aside.
Pour buttermilk into ¼ cup measure. Set aside.
Place vinegar in small bowl. Set aside.
Cream butter and sugar in stand mixer fitted with the paddle attachment at medium speed until light and fluffy.
Add egg to batter. Mix for 30 seconds until combined.
Add vanilla and red food coloring. Scrape down sides of bowl.
Turn mixer speed to low. Add half of flour mixture. Add buttermilk. Repeat. Let combine for 30 seconds. Scrape down sides of bowl.
Add baking soda to vinegar. Stir. Immediately add to batter. Turn mixer speed to high for 15 seconds.
Portion out batter into muffin tins using ¼ cup measure.
Bake for 15 minutes or until tops bounce back slightly when touched. Remove from oven.
Let cool on cooling rack for 10 minutes, then remove from tin to let them cool completely.