Position oven rack in bottom third of oven and heat to 325º. Line an 8- by 8-inch metal baking pan with foil. Press foil firmly against the sides of the pan and leave a 2-inch overhang. Coat foil with nonstick spray and set pan aside.

In a medium saucepan, melt butter over medium heat. Once butter has melted, continue cooking for 5 minutes, stirring often, until butter has stopped foaming and has turned a light brown color. Remove from heat and immediately add sugar, cocoa, vanilla, water, espresso powder (if using) and salt. Stir until combined and allow to cool for 5 minutes.

Add eggs to chocolate mixture one at a time, whisking well after each addition. When mixture is thick and shiny, add flour and whisk until blended. Continue whisking vigorously for 60 strokes. Stir in nuts and pour batter into pan.

Bake for 30-35 minutes, until a toothpick comes out with only a few moist crumbs attached. Cool completely in pan set on a wire rack. Using the foil, lift brownies from pan. Cut into 16 pieces. Serve immediately or store at room temperature in an airtight container for 2 days.