3 ounces unsweetened chocolate
1/2 cup butter
1 cup sugar
2 eggs
1 teaspoon vanilla extract
2/3 cup flour
1/4 teaspoon baking soda

Heat oven to 350°F. Line 8x8 inch baking pan with foil and spray foil with nonstick cooking spray. Melt butter and chocolate together, stirring till smooth. Remove from heat. Cool 5 minutes. Stir sugar into mixture till dissolved. Add eggs, one at a time, beating just till incorporated. Stir in vanilla. Stir dry ingredients together and stir into mixture till just combined. Spread batter in prepared pan. Bake 35 minutes till edges are set and begin to pull away from pan sides. Cool in pan on rack.
Note
Add up to 1 1/2 cups of additional ingredients such as nuts, chocolate chips, dried fruit, chopped candy, etc.