PREHEAT oven to 375° F. Grease or paper-line miniature muffin pans.
MICROWAVE first addition of morsels and butter in medium, microwave-safe bowl on HIGH power; stir. Microwave at additional 10 to 20-second intervals, stirring until smooth. Cool for 10 minutes.
ADD sugar, eggs and vanilla to melted chocolate; stir well. Gradually stir in flour until smooth. Spoon into prepared pans, filling half full.
BAKE for 12 to 14 minutes or until wooden pick inserted in center comes out clean. Immediately press one of the desired fillings (3 miniature marshmallows, 1 maraschino cherry, 5 morsels or 2 mint quarters) into center of each tart.
BAKE for additional 2 minutes. Cool in pan on wire rack for 5 minutes.
COMBINE sweetened condensed milk and remaining 1 cup morsels in a small saucepan; cook over medium-low heat, stirring constantly, for 2 to 3 minutes, or until smooth. Top each tart with about 1 teaspoon mixture. Refrigerate for at least 1 hour. Store in airtight container for up to 3 days.