Preheat oven to 325*F. Greade a 9x9-in. Metal baking pan lightly or line with parchament paper. Melt chocolate in double boiler, over simmering but not boiling water. Cool to room temperature. Cream butter until fluffy. Add sugar in thirds continuing to beat the mixture until light and fluffy. Add thr eggs, 1 at a time beating each in well. Mix in melted chocolate. Fold in flour by hand. Do not overmix. Fold in the nuts if using them. Pour batter into prepared pan. Bake for 22 to 25 minutes. If you are using a glass bking dish bake 17 to 20 minutes. Until top is dry and a toothpick in center comes out clean. Cool completely in pan before cutting.