Yield: ~ 2 dozen 

1 cup (2 sticks/228 grams) unsalted butter, at room temp

3/4 cup (150g) granulated sugar

3/4 cup (165g) firmly packed light brown sugar

2 large eggs

1/2 teaspoon pure vanilla extract

1 cup (140g) unbleached all-purpose flour

1 cup (150g) bread flour

1 teaspoon baking soda

1/2 teaspoon kosher salt

9 ounces (255g) semisweet chocolate, chopped (about 1 1/2 cups)

2 1/2 ounces (70 grams) milk chocolate, chopped (about 1/2 cup)

In the bowl of a stand mixer fitted with the paddle, cream together the butter, sugar and brown sugar on medium speed for about 5 minutes, or until light and fluffy. Stop the mixer a few times to scrape down the sides and bottom of the bowl as needed to release clinging butter or sugar. Beat in the eggs and vanilla on medium for 2 to 3 minutes or until thoroughly combined. Scrape down the bowl and paddle to make sure the eggs are fully mixed in.

In a medium bowl, stir together both flours, baking soda, and salt until well combined. Add the semi sweet and milk chocolate pieces and toss to combine.  On low speed (or with a wooden spoon), slowly add the flour-chocolate mixture to the butter-sugar mixture.  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 rest in the fridge overnight (or at least 3-4 hours) before baking.  When you are ready to bake, position a rack in the center of the oven and preheat the oven to 350F.

Drop the dough in 1/4 cup balls onto a parchment lined baking sheet about 2 inches apart. Flatten each cookie slightly with the palm of your hand. Bake for 15 to 18 minutes, or until golden on the edges but still soft in the center. I rotated the pans halfway through baking. Don’t let them brown through or you will lose the chewiness of the slightly underbaked centers.  Let cool on the baking sheet on a wire rack for 5 to 10 minutes.  Then, transfer the cookies with a spatula to the wire rack to cool completely.