Heat oven to 350°F. Spray cookie sheets with cooking spray. In medium bowl, stir cookie dough, egg yolk and flour until well mixed.
Pour sugar crystals into small bowl. Shape dough into 28 (1-inch) balls; roll in sugar crystals. Insert 1 pretzel piece into each ball for pumpkin stem. Using toothpick or tip of teaspoon, make lines around sides for pumpkin ridges. Place balls 2 inches apart on cookie sheets.
Bake 9 to 11 minutes or just until set in center when touched with fingertip. Cool 2 minutes; remove from cookie sheets to cooling racks. Cool completely, about 10 minutes.