MICROWAVE 3/4 cup morsels in a small, uncovered, microwave-safe bowl on HIGH (100%) power for 1 minute; STIR. Morsels may retain some of their original shape. If necessary, microwave at additional 10 to 15-second intervals, stirring just until morsels are melted. Cool to room temperature. Combine flour, baking soda and salt in a small bowl.
BEAT butter, brown sugar, sugar and vanilla extract in a large mixing bowl until creamy. Beat in melted chocolate and egg. Gradually beat in flour mixture. Stir in 3/4 cup morsels and nuts. Drop by rounded tablespoon onto ungreased baking sheets.
BAKE for 8 to 9 minutes or until edges are set but centers are still slightly soft. Cool on baking sheets for 3 minutes; remove to wire racks to cool completely. Thinly frost centers of cookies with chocolate frosting.
FOR CHOCOLATE FROSTING:
MICROWAVE remaining 1/2 cup morsels and butter in medium, uncovered, microwave-safe bowl on HIGH power for 30 seconds; stir until smooth. Microwave at additional 10 to 15-second intervals, stirring until smooth. Add powdered sugar and milk; stir until smooth.