Preheat the oven to 375 degrees F. Line a rimmed baking sheet with parchment.
Whisk together the flour, baking soda and salt in a small bowl; set aside.
Microwave the butter in a large microwave-safe bowl until melted, about 1 minute. Whisk in the dark brown sugar and granulated sugar until smooth; whisk in the egg and vanilla until blended. Stir in the flour mixture until just incorporated. Stir in the bittersweet, semisweet and white chocolate chips.
Transfer the dough to the center of the prepared baking sheet. Spread the dough into an 8-inch circle about 1/2 inch thick. Bake the cookie, rotating halfway through, until the top is golden brown, the edges are set and the center is still soft, 22 to 25 minutes. Cool for a few minutes on the baking sheet, then use a large spatula to transfer the cookie to a wire rack to cool. Slice the cookie into wedges.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
Copyright 2018 Television Food Network, G.P. All rights reserved.