Preheat oven to 350 degrees.
In medium saucepan melt butter and unsweetend chocolate over low heat, stirring constantly.
Remove from heat; cool.
Grease 8x8x2 or 9x9x2 inch baking pan.
Stir sugar into chocolate.
Add eggs one at a time, beat with spoon until combined. Stir in vanilla.
In another bowl, stir in the flour and baking soda.
Then, add flour mixture into chocolate mixture; stir until combined.
Then, if desired, stir in nuts.
Spread batter into pan.
Bake 30 minutes for an 8 inch pan or 25 minutes for 9 inch pan.
Cool. Frost with Chocolate Cream Cheese Frosting.