HEAT oven to 350ºF.
LINE an 8 by 8-inch pan with foil, leaving a 2-inch overhang. Spray with cooking spray.
MELT butter and chocolate in saucepan over medium heat, stirring constantly. Once the chocolate is fully melted and smooth, remove from the heat.
STIR in sugars, eggs, salt and vanilla. Whisk until smooth. Add flour; stir until thoroughly combined. Stir in 1/2 cup of the nuts. Spread into the pan and sprinkle with remaining nuts.
BAKE for 30 min. Remove from oven and let cool completely on wire rack.
