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Melt the 6 ounces (170 g) of 50% to 55% semisweet chocolate along with the oil and butter either in a microwave safe bowl or over simmering water. Whisk together until smooth and cool. This cooling part is very important and it is easy to want to rush. Be patient! Whisk the mixture from time to time to dissipate the heat.
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Meanwhile, position two racks in the upper and lower third of oven. You will be baking a total of three sheet pans of cookies, but if you try and bake them all at once, they will not bake evenly. Baking two at a time, and then the last one on its own is the way to go. Line three half-sheet pans with parchment paper; set aside. Preheat oven to 350°F/180°C.
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Beat brown sugar, eggs and vanilla (and espresso powder, if using) in a mixing bowl with an electric mixer until lightened and fluffy. Use a combination of a whisking motion and a folding motion to add the cooled chocolate mixture until combined. Sprinkle flour, baking powder and salt over this batter and fold in until a few floury streaks remain. Add the 6 ounces (170 g) of 60% to 70% chopped bittersweet chocolate and fold in until everything is combined.
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Dole out 10 brookies on each pan; each should be about the size of a walnut in the shell. Space them evenly.
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Bake for about 10 to 12 minutes total (switching pans front to back, and upper to lower rack halfway through) or until cookies have spread, the tops look a bit crackled and the edges are a bit more done than the centers, which should remain a tad soft. Cool pans on racks. Brookies are ready to serve or may be stored in single layers, separated by parchment paper, in airtight containers at room temperature for 3 days.