Position rack in the center of the oven; preheat to 350 degrees F. Line a large baking sheet with parchment paper or a silicone baking mat.

Whisk all-purpose flour, whole-wheat flour, cocoa powder, baking soda and salt in a medium bowl. Beat egg whites in a large bowl with an electric mixer until foamy, about 1 minute. Beat in granulated sugar in a slow, steady stream. Scrape down the sides, then beat in brown sugar 1 tablespoon at a time. Beat until smooth, about 3 minutes. Beat in vanilla and melted chocolate. Stir in the dry ingredients with a wooden spoon until just incorporated.

Drop the batter by tablespoonfuls onto the prepared baking sheet, 1 1/2 inches apart.

Bake the cookies until flat yet springy, with slightly cracked tops, 10 to 12 minutes. Cool on the pan for 5 minutes, then transfer to a wire rack to cool completely. Let the pan cool for a few minutes before baking another batch; replace parchment paper if torn or scorched.

Tip: Two foolproof ways to melt chocolate:

1. Bring 1 inch of water to a simmer in a double boiler set over medium-high heat, then place chocolate in the top pan. (Alternatively, to improvise a double boiler, place chocolate in a heat-safe mixing bowl that fits snugly over a pan with 1 inch of simmering water.) Stir until half the chocolate has melted. Remove the bowl and continue stirring until the chocolate has fully melted. Cool for 5 minutes at room temperature.

2. Place the chocolate in a microwave-safe bowl and microwave on High for 1 minute. Stir well, then continue microwaving in 30-second increments on High until two-thirds of the chocolate has melted, stirring well after each heating. Remove the bowl and continue stirring until all the chocolate has melted. Cool for 5 minutes at room temperature.

Enhancements: Stir 2/3 cup raisins, dried cherries or dried blueberries into the Chewy Chocolate Cookies batter along with the flour mixture. Or use 1/2 cup finely chopped pecans or walnuts.