Preheat and prep: Heat your oven to 350°F (175°C). Line two baking sheets with parchment paper.

Whisk dry ingredients: In a medium bowl, whisk 1 2/3 cups (200 g) all-purpose flour, 2 tablespoons cocoa powder, 1/2 teaspoon baking soda, and 1/2 teaspoon fine sea salt. Set aside.

Combine sugars and butter: In a large bowl, whisk 1/2 cup (100 g) granulated sugar and 1/2 cup (105 g) light brown sugar with 1/2 cup (113 g) melted, slightly cooled unsalted butter until glossy and thick, about 30 seconds.

Add wet ingredients: Whisk in 1 large egg yolk, 1/2 cup (120 g) sourdough discard, 1 1/2 teaspoons vanilla extract, and 1 teaspoon vinegar until smooth.

Add color: Stir in red gel food coloring, starting with 1/2 teaspoon. Add more as needed for a deep red. The dough should be clearly red after mixing in the dry ingredients.

Bring it together: Add the dry ingredients to the wet mixture. Fold with a spatula just until no dry streaks remain. The dough will be soft but scoopable.

Optional mix-ins: Fold in 3/4 cup white chocolate chips or chunks if using.

Scoop: Use a 1.5-tablespoon cookie scoop or heaping tablespoon to portion dough onto the prepared sheets, spacing about 2 inches apart. For thicker cookies, chill the scooped dough 15–20 minutes; for thinner, chewier cookies, bake right away.

Bake: Bake 9–11 minutes, rotating the pans halfway through. The edges should be set, and the centers should still look slightly soft and puffy.

Shape, if you like: For neat, round cookies, use a large round cookie cutter or cup to scoot the edges into shape immediately after baking.

Cool: Let cookies rest on the sheet for 5 minutes, then transfer to a wire rack to cool completely. They’ll firm up as they cool and stay soft inside.