1. Heat oven to 350 degrees. Spread chopped walnuts on a baking pan; toast until fragrant, 5 to 10 minutes. Transfer to a bowl to cool.

2. Increase the oven temperature to 400 degrees. Generously butter a 9-by-13-inch baking pan, and set aside. Bring a medium sauce- pan of water to a simmer. In a heat-proof bowl, combine the chocolate and butter. Set bowl over the simmering water until the chocolate mixture has melted. Remove from heat, and set saucepan aside.

3. Combine chocolate and butter in a heat-proof bowl or the top of a double boiler. Set over a pan of simmering water until the chocolate mixture has melted. Remove from heat; set aside.

4. In bowl of an electric mixer fitted with the paddle attachment, beat the eggs, sugar, and espresso at high speed for 10 minutes. Reduce speed to low, and add the melted-chocolate mixture and the vanilla; beat until combined. Slowly add the flour and salt; beat just until incorporated.

5. Remove the bowl from mixer, and fold in the chocolate chips and toasted nuts. Pour the batter into the prepared pan. Bake until the edges are dry but the center is still soft, about 30 minutes. Remove brownie from oven, and transfer to a wire rack to cool. Cut into 3-inch squares.