8ounces(or 227 grams) milk chocolate, between 30 to 40% cacao, chopped into ½- to 1-inch pieces
1cup(8 ounces or 227 grams) unsalted butter, chopped into 1- to 2-inch pieces
2tablespoonsDutch-processed cocoa powder, sifted if lumpy
1cup(4.5 ounces or 128 grams) all-purpose flour
1cuptightly packed, (7.5 ounces or 213 grams) dark brown sugar
1cup(7 ounces or 198 grams) granulated sugar
2teaspoonspure vanilla extract
1cup(6 ounces or 170 grams) mini chocolate chips, divided into ¾ cup and ¼ cup portions
Position a rack in the center of the oven and preheat the oven to 350°F. Lightly spray a 9 x 13-inch cake pan with cooking spray and line with parchment paper, leaving a 2-inch overhang on the pan’s two long sides. Spray the parchment, too.
Place the milk chocolate, butter, and cocoa powder in the top of a double boiler or in a heatproof bowl set over a medium, heavy-bottomed saucepan filled with a few inches of simmering water (be sure the bottom of the bowl does not touch the water). Cook over medium heat to melt the butter, using a heatproof rubber spatula to stir the mixture and scrape the sides of the bowl occasionally, until the butter and chocolate have melted and combined, about 10 minutes. Set the double boiler or bowl on a wire rack and let the chocolate mixture cool while you prep the other ingredients.
In a small bowl, whisk together the flour and salt.
In the bowl of a stand mixer fitted with the whisk attachment, combine the eggs, sugars, and vanilla. Whisk on low until combined, 1 to 2 minutes, using a rubber spatula to scrape down the bottom and sides of the bowl as necessary. Increase the mixer to medium and slowly pour in the chocolate mixture, aiming for the sides of the bowl as opposed to the whisk. Continue whisking on medium until the batter is smooth, about 1 minute.
Sprinkle the dry ingredients over the batter all at once and use a rubber spatula to mix until just combined—at this point, it’s okay to have one or two small streaks of flour left in the batter. Mix in ¾ cup of the mini chocolate chips.
Pour the batter into the prepared pan and use an offset spatula to smooth the top. Sprinkle the remaining mini chocolate chips over the batter. Bake for 35 to 40 minutes, or until a skewer inserted into the center of the brownies comes out with a few crumbs attached. Cool completely on a wire rack before slicing. It’s better to pull the brownies out of the oven early than leave them in too long—if you overbake the brownies, they’ll be tough. They might appear under baked, but I promise that when they’ve cooled, they will be perfect.
Run a butter knife or an offset spatula along the edges of the pan and use the overhanging parchment as handles to lift the brownies out of the pan and onto a cutting board. Slice into 20 rectangles, each about 2 ¼ inches wide and 2 ½ inches long, and serve. The brownies can be stored in an airtight container or zip-top bag at room temperature for up to 3 days.