1. Beat 3/4 cup butter and cream cheese in medium bowl; add flour and 1/4 cup sugar, beating until well blended. Cover; refrigerate about one hour or until dough is firm. Shape dough into 1-inch balls; press each ball onto bottom and up sides of about 36 small muffin cups (1-3/4 inches in diameter).
2. Heat oven to 350°F. Combine egg, remaining 1/2 cup sugar, melted butter, lemon juice and vanilla in small bowl; stir until smooth. Stir together milk chocolate chips and peanut butter chips. Set aside 1/3 cup chip mixture; add remaining chips to egg mixture. Evenly fill muffin cups with egg mixture.
3. Bake 20 to 25 minutes or until filling is set and lightly browned. Cool completely; remove from pan to wire rack.
4. Combine remaining 1/3 cup chip mixture and shortening in small microwave-safe bowl. Microwave at MEDIUM (50%) 30 seconds; stir. If necessary, microwave at MEDIUM an additional 10 seconds at a time, stirring after each heating, until chips are melted and mixture is smooth when stirred. Drizzle over tops of tassies. 3 dozen cookies