How to Make It
Preheat the oven to 350°. Line 2 large cookie sheets with parchment paper. In a medium bowl, sift the flour with the cocoa, baking soda and salt.
In a standing electric mixer fitted with a paddle, beat the butter with the granulated and light brown sugars at medium speed until light and fluffy, about 3 minutes. Beat in the eggs and vanilla. Add the dry ingredients and beat at low speed until incorporated. At low speed, beat in the bittersweet- and white-chocolate chunks until they are evenly distributed.
Scoop eight 1/4-cup mounds of batter onto each of the prepared cookie sheets, leaving 3 inches between them. Flatten the mounds into 2-inch rounds. Bake on the lower and middle racks of the oven for 20 minutes, until the cookies rise and then flatten slightly; switch the cookie sheets halfway through baking. Let the cookies cool for 10 minutes, then slide the parchment paper onto racks and let the cookies cool completely. Line the cookie sheets with fresh parchment and repeat to make the remaining cookies.