Heat oven to 375°F. Grease 13x9-inch pan with shortening or cooking spray. In 1-quart saucepan, melt 3/4 cup butter and the chocolate over low heat, stirring constantly, until smooth. Remove from heat. Stir in 1 tablespoon vanilla; set aside.
In large bowl, beat granulated sugar and eggs with electric mixer on medium speed 3 to 4 minutes or until sugar is dissolved. Stir in flour, chocolate mixture and nuts just until well blended. Pour batter into pan.
Bake 25 to 35 minutes. DO NOT OVERBAKE. Cool completely, about 1 hour.
In small bowl, beat frosting ingredients with electric mixer on low speed until smooth; spread over cooled bars. Refrigerate 1 hour. Cut into 6 rows by 4 rows. If desired, garnish each bar with pecans.