Heat oven to 350'F. In small microwave-safe bowl, place quarter cup chocolate chips. Microwave at HIGH (100%) 20-30 seconds or just until chocolate is melted and smooth when stirred; set aside to cool slightly. In large mixer bowl, combine butter, shortening, sugar, reserved melted chocolate, egg yolk and vanilla; blend well. Stir in flour and salt. Roll dough into 3 cm balls. With fork, slightly beat egg white. Dip each ball into egg white, roll in chopped nuts. Place balls on ungreased cookie sheet, about 3 cm apart. Press center of each ball with thumb to make indentation. Bake 10-12 minutes or until set. Remove from oven; immediately place several of remaining 0.75 cup chocolate chips in center of each cookie. Carefully remove from cookie sheet to wire rack. After several minutes, swirl melted chocolate in each thumbprint. Cool completely. Makes about 2.5 dozen cookies.