PREHEAT oven to 350° F.
BEAT butter and powdered sugar in large mixer bowl until creamy. Beat in vanilla extract. Gradually beat in flour and nuts. Stir in 1 1/2 cups morsels. Roll dough into 1-inch balls; place on ungreased baking sheets.
BAKE for 10 to 12 minutes or until set and light golden brown on bottom. Cool for 2 minutes on baking sheets; remove to wire racks to cool completely.
MICROWAVE remaining morsels in heavy-duty plastic bag on HIGH (100%) power for 30 to 45 seconds; knead. Microwave at additional 10- to 20-second intervals, kneading until smooth. Cut tiny corner from bag; squeeze to drizzle over cookies. Refrigerate cookies for about 5 minutes or until chocolate is set. Store at room temperature in airtight containers.
NOTE: For a spicier cookie, add 2 to 2 1/2 teaspoons ground cinnamon to flour before adding to butter-sugar mixture.