Stir together butter, sugar and milk in heavy 2-1/2 to 3-quart saucepan; bring to boil, stirring constantly. Boil 5 minutes over medium heat, or until candy thermometer reaches 234 °F, stirring constantly to prevent scorching. Remove from heat. Gradually stir in chocolate until melted. Add remaining ingredients; mix until well blended. Pour into greased 9 x 9-inch or 13 x 9-inch pan. Cool at room temperature; cut into squares.