Sift or whisk together cake flour, cocoa powder, baking powder, and a pinch of salt until evenly combined.
Beat softened butter with caster sugar until pale and fluffy.
Beat in the eggs one at a time until smooth.
Mix in buttermilk, vegetable oil, vanilla extract, red food coloring, and white vinegar until fully combined.
Add the dry mixture and mix just until no dry flour remains (don’t overmix).
Cover the bowl and refrigerate the batter (assumption: 20–30 minutes) — this is the corrected Step 4.
Preheat oven (assumption: 350°F / 175°C). Line a muffin tin with paper liners and fill each about 2/3 full.
Bake (assumption: 17–20 minutes) until a toothpick comes out with a few moist crumbs. Cool completely.
Frosting: Beat butter and cream cheese until smooth. Add vanilla and salt. Gradually add powdered sugar and beat until fluffy.
Pipe or spread frosting on fully cooled cupcakes and serve.