In a large bowl, cream butter and brown sugar until light and fluffy. Beat in egg yolk and vanilla. In a small bowl, whisk flour, salt and baking powder; gradually beat into creamed mixture.
Roll dough between two sheets of waxed paper into a 12x10-in. rectangle; transfer to a baking sheet. Refrigerate 30 minutes.
In a small microwave-safe bowl, melt chocolate chips and shortening; stir until smooth. Stir in walnuts, milk and vanilla. Remove top sheet of waxed paper; spread dough with filling. Using waxed paper to shape dough, roll up tightly jelly-roll style, starting with a long side. Wrap in plastic; refrigerate 2 hours or until firm.
Preheat oven to 375°. Unwrap and cut dough crosswise into 1/4-in. slices. Place 2 in. apart on greased baking sheets. Bake 7-9 minutes or until set. Cool 1 minute before removing from pans to wire racks.
For food safety reasons, use only commercially prepared cookie dough.
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