Divide the dough in half and add chocolate and vanilla to 1 half and incorporate with hands. Add egg yolk, peppermint extract, and crushed candy to other half of dough and incorporate with hands. Cover both with plastic and chill for approximately 5 minutes. Roll out doughs separately to approximately 1/4-inch thickness. Place peppermint dough on top of chocolate and press together around the edges. Using waxed paper or flexible cutting board underneath, roll dough into log. Wrap in wax paper and refrigerate for 2 hours.
Preheat oven to 375 degrees.
Remove dough from the refrigerator and cut into 1/2-inch slices. Place cookies 1-inch apart on greased baking sheet, parchment, or silicone baking mat and bake for 12 to 13 minutes, rotating the pan halfway through cooking time. Remove from oven and let sit on baking sheet for 2 minutes, then move to a wire rack to cool completely. Store in an airtight container for up to 1 week.
Sift together flour, baking powder, and salt. Set aside. Place butter and sugar in large bowl of electric stand mixer and beat until light in color. Add egg and milk and beat to combine. Put mixer on low speed, gradually add flour, and beat until mixture pulls away from the side of the bowl. Divide the dough in half, wrap in waxed paper, and refrigerate for 2 hours.
Preheat oven to 375 degrees F.
Sprinkle surface where you will roll out dough with powdered sugar. Remove 1 wrapped pack of dough from refrigerator at a time, sprinkle rolling pin with powdered sugar, and roll out dough to 1/4-inch thick. Move the dough around and check underneath frequently to make sure it is not sticking. If dough has warmed during rolling, place cold cookie sheet on top for 10 minutes to chill. Cut into desired shape, place at least 1-inch apart on greased baking sheet, parchment, or silicone baking mat, and bake for 7 to 9 minutes or until cookies are just beginning to turn brown around the edges, rotating cookie sheet halfway through baking time. Let sit on baking sheet for 2 minutes after removal from oven and then move to complete cooling on wire rack. Serve as is or ice, as desired. Store in airtight container for up to 1 week.
Yield: approximately 3 dozen, 2 1/2-inch cookies
Prep Time: 15 minutes
Cook Time: 7 to 9 minutes
Inactive Prep Time: 2 hours
Recipe courtesy of Alton Brown