In a large bowl, cream butter and sugar until light and fluffy. Beat in eggs. Beat in chocolate and vanilla until well blended. Combine flour and baking powder; stir into chocolate mixture. Fold in pecans. Pour into a greased 13-in. x 9-in. baking pan.
In another large bowl, combine cream cheese and butter. Beat in the sugar, egg, flour and vanilla until well combined. Fold in pecans. Spread over the chocolate layer; sprinkle with chips.
Bake at 350° for 20-25 minutes or until edges pull away from sides of pan. Sprinkle with marshmallow; bake 2 minutes longer or until puffed. Spread evenly over cream cheese layer. Cool on a wire rack.
In a large saucepan, combine first four topping ingredients. Cook and stir over low heat until smooth. Transfer to a large bowl. Add the confectioners' sugar and vanilla; beat until smooth. Spread over cooled bars. Store in the refrigerator.