Melt butter and chocolate in a small saucepan over low heat, stirring constantly. Remove from heat, and cool.
Combine sugar and eggs in a medium mixing bowl; beat with an electric mixer until blended. Add chocolate mixture and flour; beat just until smooth. Stir in pecans and vanilla.
Pour batter into a greased 8- inch square baking pan. Bake at 350° for 20 to 25 minutes. Cool in pan. Spread with Chocolate Frosting; cut into 2-inch squares.