Melt the chocolate and set aside to cool. Cream together butter, vanilla, and sugar until light and fluffy. Add the eggs one at a time, beating well after each. Add flour and salt and mix until blended. Stir in the nuts and divide the batter in half and add the cooled chocolate to one half. Drop the batters alternately by teaspoonfuls into a greased pan 13 x 9 x 2”. Run a knife through the batter several times to marbelize. Bake in a moderate oven (350 degrees F) about 45 minutes. Cool and frost with chocolate frosting. Cut into squares.