Combine shortening, 1/3 cup butter, egg yolks, and vanilla in large bowl. Mix at low speed until blended. Set aside cocoa packet from cake mix. Gradually add cake mix, blend well. Divide dough in half. Add cocoa packet and remaining 1 tbsp butter to 1/2 of dough. Knead until well blended and chocolate colored. Roll out yellow dough into 18 X 12 X 1/8 inch rectangle. Repeat for chocolate dough. Roll between waxed paper. Remove top pieces of wax paper. Lay yellow dough directly on top of chocolate. Remove remaining layers of waxed paper. Roll up jelly roll fashion. Regrigerate 2 hours. Preheat oven to 350 degrees. Grease cookie sheets. Cut dough into 1/8 inch slices. Place sliced dough 1/2 inch apart on cookie sheets. Bake 9-11 minutes or until lightly browned. Cool 5 minutes on cookie sheets. Remove to cooling racks.
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