preheat oven to 400 degrees. melt 1/4 cup of the butter and pour into a foil lined jelly roll pan. arrange crackers on top of butter. melt remaining butter, add the brown sugar and bring to a boil over medium heat, reduce heat and cook for two minutes stirring occasionally. remove from heat, add condensed milk and mix well. spread over crackers. bake at 400 for 10-12 minutes until bubbly and slightly darkened around the edges. cool one minute,and sprinkle the chocolate chips evenly over filling. let stand about five minutes until the chips melt, then spread evenly. sprinkle with the chopped nuts and lightly press them into the chocolate. chill until set, remove foil off back and break or cut into 1 1/2 inch pieces.