Cream butter until light and fluffy.
Beat in vanilla and sugar.
Add eggs, one at a time, beating well after each addition.
Add flour and salt and mix until blended.
Stir in nuts; divide batter into halves; add cooled melted chocolate to one.
Drop batters alternately by teaspoons onto greased, floured pan (13 x 9 x 2 inches).
Run knife through batter several times to marbleize.
Bake in preheated moderate oven (350°F.) for about 45 minutes.
Turn out on rack to cool.
At serving time, cut into bars about 3 x 1 1/2 inches.
Makes 24 bars.