In a large bowl, cream butter and sugar until light and fluffy. Beat in eggs and vanilla. Combine dry ingredients; gradually add to creamed mixture. Stir in pecans. , Spread in a greased 13-in. x 9-in. baking pan. Bake at 325° for 30-35 minutes or until a toothpick comes out with moist crumbs. Cool on a wire rack., In a small saucepan, combine the sugar, butter and milk; cook and stir over low heat until sugar is dissolved. Remove from the heat; stir in chips and marshmallows until melted. Chill until mixture reaches room temperature. In a large bowl, beat until frosting reaches spreading consistency. Spread over brownies.