Preheat the oven to 400°F (200°C). Line the muffin tin with paper liners.
Melt butter in a small pan. Let cool slightly until you need it.
Dry ingredients: Whisk flour, baking powder, baking soda, sugar, and a pinch of salt in a large bowl with an egg whisk.
Wet ingredients: Whisk buttermilk, red gel food coloring, eggs, vanilla extract, vinegar, and melted butter in a medium bowl.
Combine: Pour the wet ingredients over the flour mixture. Stir gently and shortly until just combined. Don’t over stir. If you want to add chocolate, berries, or nuts, carefully fold them in now (Note 5).
Bake: Scoop the batter into the cups. Bake for about 20 minutes or until a toothpick inserted in the middle of a muffin comes out clean.
Rest for 5 minutes in the pan. Remove and let cool on a wire rack. If desired, sprinkle with icing sugar before serving.