Preheat and prep: Heat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats. This helps prevent over-browning and makes cleanup a breeze.

Whisk dry ingredients: In a medium bowl, whisk flour, cocoa powder, cornstarch, baking soda, and salt until evenly combined. Set aside.

Mix wet ingredients: In a large bowl, whisk the melted, slightly cooled butter with granulated sugar and brown sugar until glossy and smooth, about 45 seconds. Add the egg, egg yolk, and vanilla. Whisk again until the mixture looks thick and shiny.

Add color and vinegar: Stir in the vinegar. Add red gel food coloring a little at a time until the batter is a deep, vibrant red. Gel coloring works best because it won’t thin your dough.

Combine wet and dry: Add the dry ingredients to the wet mixture. Use a spatula to fold gently until just combined. Avoid overmixing—stop when you no longer see dry streaks.

Fold in chips: Stir in the white chocolate chips, reserving a small handful for topping. The dough will be soft but scoopable.

Chill briefly (optional but helpful): If your kitchen is warm, chill the dough for 15–20 minutes to prevent too much spread. This step also deepens flavor.

Scoop and shape: Scoop 1.5-tablespoon mounds (a medium cookie scoop works well) onto the prepared sheets, spacing about 2 inches apart. Press a few extra white chocolate chips on top for bakery-style looks.

Bake: Bake for 9–11 minutes, rotating pans halfway. The edges should look set and the centers still slightly soft and puffy. Don’t overbake—carryover heat will finish them.

Finish and cool: If you like, sprinkle a pinch of flaky sea salt on each cookie as soon as they come out. Let them cool on the tray for 5 minutes, then transfer to a rack to cool completely.