Preheat oven to 325 degrees. In a medium mixing bowl, combine peanut butter, sugar, flour and egg whites; stir in 3/4 cup of the walnuts. Spread mixture in a lightly greased 9-by-13-inch pan. Bake for 10 to 12 minutes or until lightly browned along the edges. Meanwhile, break chocolate into 1 to 2 inch pieces. Remove cookies from oven; immeadiately place chocolate on top of cookies. Return to oven and cook 1 minute more. Remove from oven; spread melted chocolate over tops of the cookies and then sprinkle with the remaining walnuts. Cut into squares while warm.