from Flour: Spectacular Recipes from Boston's Flour Bakery & Cafe, by Joanne Chang, 2010
1 cup unsalted butter, at room temperature
3/4 cup granulated sugar
3/4 cup firmly packed light brown sugar
1/2 teaspoon vanilla extract
1 cup unbleached, all-purpose flour
1 cup bread flour
1 teaspoon baking soda
1/2 teaspoon Kosher salt
9 ounces semisweet chocolate (about 1 1/2 cups)
2 1/2 ounces milk chocolate, chopped (about 1/2 cup)
Using a stand mixer fitted with the paddle attachment (or a handheld mixer or a wooden spoon), cream together the butter and both sugars on medium speed for about 5 minutes or until mixture is light and fluffy. (If using a handheld mixer or wooden spoon it will take more like 10 minutes.) Stop the mixer a few times and use a rubber spatula to scrape the sides and bottom of the bowl and the paddle to release any clinging butter or sugar. Beat in the eggs and vanilla on medium speed for 2-3 minutes, or until thoroughly combined. Scrape the bowl and paddle again to make sure the eggs are thoroughly incorporated.
In a medium bowl, stir together both flours, baking soda, and salt until well mixed. Add both chocolates and toss to combine. On low speed (or with the wooden spoon), slowly add the flour-chocolate mixture to the butter-sugar mixture and then mix just until the flour mixture is totally incorporated and the dough is evenly mixed.
For the best results, scrape the dough into an airtight container and let it rest in the fridge overnight (or for at least 3-4 hours) before baking. When you are ready to bake, position a rack in the center of the oven, and heat to 350 F.
Line two baking sheets with parchment paper. Drop the dough in 1//4 cup balls (I used a regular sized ice cream scoop), spacing them about 2 inches apart. Flatten each ball slightly with the palm of your hand.
Bake for 15-18 minutes, or until the cookies are golden brown on the edges and slightly soft in the center. DO NOT let them get brown all the way through and over bake, you want the center be slightly underbaked and chewy. Let cool on the baking sheet on a wire rack for 5-10 minutes, then transfer the cookies to a rack to cool completely.
Yield: 24 cookies