Preheat the oven to 350°F. Line two baking sheets with parchment paper.
Whisk flour, cocoa, baking soda, cornstarch, and salt in a medium bowl.
Cream butter, granulated sugar, and brown sugar until light and fluffy.
Beat in liquid eggs, vanilla, red food coloring, vinegar, and buttermilk until smooth.
Add dry ingredients to the bowl. Mix on low just until the dough comes together.
Fold in white chocolate chips by hand until evenly distributed.
Chill the dough for about 15 minutes to firm slightly for easier scooping.
Scoop 1½-tablespoon portions onto the sheets, spacing about 2 inches apart.
Bake 10–12 minutes, until edges set and centers look soft and puffy.
Cool on the sheet 5 minutes, then move cookies to a rack to finish cooling.