1 ½ lb semisweet chocolate, chopped
1 cup all-purpose flour
1 Tbsp baking powder
1 tsp salt
6 large egg
2 ½ cup sugar
2 tsp pure vanilla extract
2 tsp strong-brewed espresso
6 oz semisweet chocolate chips, 1 cup
1 cup walnut halves, lightly toasted and coarsely chopped optional
1. Preheat the oven to 350°. Spray a 9-by-13-inch glass or ceramic baking dish with cooking spray. In a double boiler, melt the butter with the chopped chocolate over low heat. In a small bowl, whisk the flour with the baking powder and salt.
2. In a large bowl, using a handheld mixer, beat the eggs with the sugar at medium-high speed until pale and thick, 4 minutes. Beat in the vanilla and espresso. Add the flour mixture and beat until just incorporated. Beat in the melted chocolate at medium speed. Using a spoon, stir in the chocolate chips and walnuts.
3. Scrape the batter into the prepared baking dish. Bake in the center of the oven for about 1 hour, until the top is shiny and lightly cracked, the edges are set and the center is still a bit jiggly. Transfer the dish to a rack and let cool completely. Cut the brownie into 12 large rectangles and serve.