Place rack in bottom 1/3 of oven and preheat to 325°.
Line a 8x8 inch metal pan with foil leaving a 2 in. overhang. Spray with cooking spray.
Melt butter in medium pan until butter melts. Remove from heat and pour butter into a mixing bowl.
Add sugar, cocoa, 2 tsp water, vanilla, and salt. Stir to blend. Let cool 5 minutes.
Add eggs one at a time to hot mixture and beat after each addition.
When mixture is thick and shiny, add flour and stir. Beat for a minute or two, stir in optional walnuts.
Transfer to pan. Bake until toothpick comes out almost clean (about 25-35 minutes) - the center will still be a bit soft. Cool on rack.
Remove brownies by lifting up on foil. Cut into small pieces - 36 squares.