Preheat oven to 350° F. Line an 8 inch x 8 inch cake pan with foil and lightly grease the foil.
In a small pan melt chocolate and butter together over low heat stirring until smooth. Remove from heat. Add the sugar and stir until combined.
In a large bowl beat eggs and vanilla together. Add chocolate-sugar mixture to eggs.
Sprinkle flour over chocolate mixture. Fold in flour and milk until combined.
Fold in nuts if using.
Pour into prepared pan.
Bake in center of oven for 22-25 minutes until brownie comes away from the sides of the pan but is still moist and fudgy in the center. Cool completely on a wire rack. Refrigerate before cutting.