Prep the pan and oven: Heat oven to 350°F (175°C).
Lightly grease a 24-cup mini muffin pan or line with mini paper liners. Greasing helps keep edges chewy, while liners make cleanup easy.
Mix the wet ingredients: In a medium bowl, whisk the melted butter, granulated sugar, and brown sugar until glossy and combined. Add the eggs one at a time, whisking well after each.
Stir in vanilla, buttermilk, vinegar, and food coloring.
Whisk the dry ingredients: In a separate bowl, whisk flour, cocoa powder, baking soda, and salt. Break up any cocoa lumps for a smooth batter.
Combine without overmixing: Add the dry mixture to the wet. Use a spatula to fold until just combined and no dry streaks remain.
The batter will be thick but scoopable.
Portion the batter: Fill each mini muffin cup about 3/4 full. A small cookie scoop makes this quick and tidy.
Bake: Bake for 9–12 minutes, until the tops are set and a toothpick comes out with a few moist crumbs. Do not overbake or you’ll lose the fudgy center.
Cool: Let the brownie bites cool in the pan for 5 minutes, then gently remove to a wire rack to cool completely.
Make the glaze (optional): Beat cream cheese and butter until smooth.
Add powdered sugar, vanilla, and a pinch of salt. Beat until fluffy, adding milk a teaspoon at a time until it’s drizzleable or pipeable.
Finish and serve: Pipe a small swirl on each bite or drizzle lightly. Add a few chocolate flakes or red sugar sprinkles if you want a little sparkle.