In a small mixing bowl, cream butter and sugar. Beat in egg yolk and vanilla. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk. Stir in mint. Shape into two 8-in. rolls; wrap each in plastic wrap. Refrigerate for 2 hours or until firm.
Unwrap and cut into 1/4-in. slices. Place 1 in. apart on greased baking sheets. Bake at 350 F. for 8-12 minutes or until edges are golden. Remove to wire racks to cool.
In a microwave-safe bowl, melt the chocolate chips and shortening; stir until smooth. Dip each cookie halfway into chocolate; shake off excess. Place on waxed paper to harden. Yield: about 4-1/2 dozen.
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