Butter a 15"x10" jelly roll pan; set aside. Combine sugar, salt, water and butter in a heavy 3-quart saucepan. Heat to boiling on medium-high heat. Add 3/4 cup almonds and cook, stirring constantly, to hard crack stage (300-310 degrees). Remove from heat. Stir in remaining 3/4 cup almonds, 1/2 cup walnuts, and rum extract, mixing only enough to combine. Very quickly, pour into prepared pan. Spread evenly. Place chocolate on warm toffee until melted. Spread chocolate evenly over toffee. Sprinkle with remaining 1/2 cup walnuts. Cool; break into pieces.