Microwave semisweet chocolate morsels in a microwave-safe bowl on high for one minute, or until smooth, stirring every 30 seconds.
Beat butter and sugar at medium speed with an electric mixer until creamy; add eggs, one at a time, beating well after each addition. Beat in vanilla and melted chocolate.
Combine flour, baking powder and salt; gradually add to chocolate mixture, beating until well blended. Stir in chopped pecans and milk chocolate chips. Drop dough by heaping tablespoonfuls onto parchment paper-lined baking sheets. Press 3 marshmallows into each cookie.
Bake at 350° for 10 to 12 minutes, or until set. Remove to wire racks to cool completely. Makes about 3 dozen.