Preheat oven to 350° for a metal pan or 325° for a glass pan.

In a heavy small pan over very low heat or a double boiler, melt chocolate and butter or margarine. Stir occasionally until well blended. Remove from heat and let cool to room temperature. Beat eggs, sugar, salt and vanilla extract together. Beat in chocolate mixture and mix in flour. Fold in nuts if you are using them.

Spray or lightly butter pan and spread in batter. If you use an 8 x 8 pan, bake in center of oven 20-25 minutes or until the top is dry and a wooden toothpick inserted near the center comes out barely moist. If you use a 9 x 9 pan, bake 18-22 minutes. Cool to room temperature in the pan on a rack before cutting into desired size.