½cup(4 ounces) unsalted butter, cut into 1-inch cubes
1⅔cup(7.5 ounces) all-purpose flour
¾cuptightly packed (5.65 ounces) dark brown sugar
¼cup(1.75 ounces) granulated sugar
2teaspoonspure vanilla extract
4ouncesdark chocolate (at least 70% cocoa), from whole fèves or a high quality chocolate bar, chopped into ½ to 1-inch pieces
4ouncesmilk chocolate (at least 35% cocoa), from whole fèves or a high quality chocolate bar, chopped into ½ to 1-inch pieces
In a light-colored saucepan, melt the butter over medium-low heat, swirling the pan occasionally, until it starts to foam. Use a heatproof rubber spatula to scrape the bottom and sides of the pan occasionally to prevent the milk solids in the butter from burning. Cook for 5 minutes, or until the butter smells nutty and is amber with dark flecks at the bottom of the pan. Remove from the heat and immediately pour the brown butter into a liquid measuring cup. Set it on a wire rack to cool while you prep the other ingredients.
Position a rack in the center of the oven and preheat the oven to 350°F. Line two half sheet pans with parchment paper.
Pour the brown butter over the sugar in the mixer bowl and beat on low until just combined. Add the egg and vanilla and beat until just combined. Use a rubber spatula to scrape down the bottom and sides of the bowl. With the mixer on low, gradually add the dry ingredients and beat until just combined. Scrape down the bottom and sides of the bowl once more, and beat on low for an additional 30 seconds. Increase the mixer to medium- low, add the chocolates all at once, and beat until the chocolate is evenly distributed throughout, about 1 minute.
Use a 3-tablespoon cookie dough scoop to portion the cookie dough into balls, placing them at least 3 inches apart on the prepared sheet pans. Bake one pan at a time for 10 minutes, or until the edges have set but the centers are still gooey. The cookies will look puffed when you pull them out of the oven, but will fall and crack into the perfect cookies as they cool. Cool the cookies on the pan on a wire rack for 20 minutes, or until the edges and bottoms of the cookies have set and feel firm to the touch. Repeat with the remaining cookie dough (or freeze it to bake later). Serve warm or at room temperature. The cookies can be stored in an airtight container or zip-top bag at room temperature for up to 3 days.