1. Adjust oven rack to lowest position and preheat oven to 350 degrees. Make foil sling for13 by 9 inch baking pan by folding 2 long sheets of aluminum foil: first sheet should be13 inches wide and second sheet should be 9 inches wide. Lay sheets of foil in panperpendicular to each other, with extra foil hanging over edges of pan. Push foil intocorners and up sides of pan, smoothing foil flush to pan. Grease foil

.2. Whisk flour, baking powder and salt together in bowl. Whisk boiling water, unsweetenedchocolate, cocoa and espresso powder, if using, in large bowl until well combined andchocolate is melted. Whisk in sugar, oil and vanilla. Using a rubber spatula, stir flourmixture into chocolate mixture until combined; fold in chocolate chips.

3. Scape batter into prepared pan and smooth top. Lay nuts, if using, on top of batter.Bake until toothpick inserted halfway between edge and center comes out with few moistcrumbs attached, 30-35 minutes, Let brownies cool in pan on wire rack for 2 hours.

4. Using foil overhang, lift brownies from pan, Return brownies to wire rack and let coolcompletely, about 1 hour, Cut into squares and serve. ( Brownies can be stored at roomtemperature for up to 4 days.)