1. In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, cornstarch, baking soda, and fine sea salt until well combined and no lumps remain.

2. In a large mixing bowl, beat the softened unsalted butter, light brown sugar, and granulated sugar together with a hand mixer or stand mixer on medium speed until light and fluffy, about 2–3 minutes.

3. Add the egg and egg yolk to the butter mixture and beat until smooth. Mix in the vanilla extract, red food coloring, and white vinegar or lemon juice until the mixture is evenly colored and creamy.

4. Add the dry ingredients to the wet ingredients in two additions, mixing on low speed just until no streaks of flour remain. If the dough seems very dry, you can add 1–2 teaspoons of milk, but the dough should be thick and soft, not sticky.

5. Fold in the white chocolate chips or chunks with a spatula until they are evenly distributed throughout the dough.

6. Line baking sheets with parchment paper. Use a cookie scoop or tablespoon to portion about 1 1/2 tablespoons of dough per cookie, placing the dough balls 2 inches apart on the prepared pans.

7. Bake the cookies at 350°F (175°C) for 9–11 minutes, or until the edges look set and the centers are slightly puffed but still soft. Do not overbake; the cookies will continue to set as they cool.

8. If desired, press a few extra white chocolate chips on top of each cookie immediately after removing them from the oven for a bakery-style look. Let the cookies cool on the baking sheet for 5–10 minutes, then transfer them to a wire rack to cool completely.