1In a food processor, pulse roasted pecans and 1/2 cup cinnamon pecans with both sugars using 1 second pulses, until pecans are finely ground. Add the flour and salt and process until combined. Scatter butter pieces over dry ingredients and process until mixture resembles damp sand. With machine running, add egg yolk and process about 20 seconds until dough comes together.
2Turn dough out onto a dry work surface and gently knead until it is evenly moistened. Roll dough into a 12 inch log. Cut log in half and wrap each half in plastic wrap. Place dough logs in freezer for 30 minutes. After 30 minutes and while dough is still workable, unwrap logs and roll on work surface to get rid of flat side. Rewrap dough and refrigerate until well chilled, at least 2 hours.
3When ready to bake cookies, preheat oven to 325° F. Move oven racks to center. Line 2 large baking sheets with parchment paper.
4Remove 1 roll dough from refrigerator and slice into 1/4 inch slices, turning dough 1/4 turn after each slice. Place slices on a prepared baking sheet. Cookies will not spread, so they can be placed close together. Repeat with second dough log and prepared baking sheet.
5Put both baking sheets in oven, one below the other. Bake 12 minutes, and rotate baking sheets top to bottom and front to back. At this time, place a cinnamon pecan half in the center of each cookie. Continue baking cookies for an additional 12 minutes. Remove from oven and cool on baking sheet 3 minutes; then remove to wire rack to complete cooling.