Prep the basics. Heat your oven to 350°F (175°C). Line two baking sheets with parchment paper. This helps with even baking and easy cleanup.

Mix dry ingredients. In a bowl, whisk together flour, cocoa powder, cornstarch, baking soda, and salt until no cocoa clumps remain. Set aside.

Cream butter and sugars. In a large bowl, beat the softened butter with granulated and brown sugars for 2–3 minutes until light and fluffy. This introduces air for a softer cookie.

Add wet ingredients. Beat in the egg and egg yolk, then the vanilla and vinegar. Mix until smooth, scraping down the bowl. The yolk adds richness and chew.

Add color. Mix in red gel food coloring. Start with 1 teaspoon, then add more as needed to get a deep red. Gel coloring gives strong color without watering down the dough.

Combine wet and dry. Add the dry mixture to the wet in two additions. Mix on low until just combined. Don’t overmix—stop when the flour streaks disappear.

Fold in chips. Stir in the white chocolate chips by hand. The dough should be thick and slightly sticky.

Chill (optional but helpful). For thicker cookies, chill the dough 20–30 minutes. This firms the butter and reduces spreading.

Scoop and shape. Use a 2-tablespoon scoop to portion dough onto the sheets, spacing 2 inches apart. Press a few extra white chocolate chips on top for a bakery look.

Bake. Bake 9–11 minutes, until the edges are set and the centers look slightly underdone. They’ll continue to set as they cool. For chewier cookies, err on the shorter side.

Finish and cool. If you like, sprinkle with a pinch of flaky sea salt. Let cookies cool on the sheet for 5 minutes, then transfer to a rack to cool fully.