Preheat oven to 350 °F. Butter and flour an 8 x 8 inch baking pan.
Melt chocolate with butter. Set aside.
In medium mixer bowl, beat eggs at high speed for 1 minute.
Gradually add sugar, beating at medium speed until mixture is pale yellow (5 minutes).
Beat in melted chocolate and vanilla until thoroughly blended. Mix in flour at low speed. Stir in nuts.
Pout into prepared pan. Bake for 20 minutes. Cool in pan and cut into squares.