1. Microwave semisweet chocolate morsels in a small microwave-safe glass bowl at HIGH 1 minute or until smooth, stirring every 30 seconds.
2. Beat butter and sugar at medium speed with an electric mixer until creamy; add eggs, 1 at a time, beating until blended after each addition. Beat in vanilla and melted chocolate.
3. Combine flour, baking powder, and salt; gradually add to chocolate mixture, beating until well blended. Stir in chopped pecans and 1/2 cup milk chocolate morsels.
4. Drop dough by heaping tablespoonfuls onto parchment paper-lined baking sheets. Press 3 marshmallows into each portion of dough.
5. Bake at 350° for 10 to 12 minutes or until set. Remove to wire racks.