Heat oven to 350°F Grease and flour 9-inch square pan. In 1 1/2-quart saucepan, melt 1/3 cup butter and 2 1/2 ounces of the chocolate over low heat, stirring frequently. Cool slightly, about 15 minutes.
In medium bowl, beat chocolate mixture, Bisquick mix, granulated sugar, vanilla and eggs with electric mixer on low speed 30 seconds, scraping bowl frequently. Beat on medium speed 1 minute. Spread batter in pan.
Bake 20 to 25 minutes or until toothpick inserted in center comes out clean. Remove from oven; immediately sprinkle with marshmallows. Cover; let stand about 5 minutes or until marshmallows soften. Uncover; cool completely, about 1 hour.
In 1 1/2-quart saucepan, melt remaining 2 1/2 ounces chocolate and 1 tablespoon butter; cool slightly. Stir in sour cream and powdered sugar until smooth. Spread over marshmallow layer. Cover; refrigerate at least 2 hours or until firm. For bars, cut into 6 rows by 6 rows. Store in refrigerator.