Step 1. : Preheat the oven to 350°F. Line two cookie sheets with parchment paper. On a clean work surface, knead the cookie dough with the flour until smooth. Dust the work surface with flour. Roll out the dough to a scant 1/4-inch. Using the template, cut out as many celery shapes as possible. Transfer cutouts to the prepared pans about 2 inches apart. Cut the slices in the tops of the celery cookies and spread them open slightly. Repeat with the remaining dough and scraps. Place the cookies in the oven and bake until they are just golden, about 10 to 12 minutes. Remove the cookies from the oven, transfer them to a wire rack and let them cool completely.

Step 2. : Heat the corn syrup in a small bowl in the microwave for about 5 to 10 seconds or until bubbly. Working with one cookie at a time, lightly brush the top of the cookie with the heated corn syrup and sprinkle it with decorating sugar. Repeat with the remaining cookies.

Step 3. : For the frosting, place the confectioners sugar, egg white powder and 5 teaspoons warm water into a bowl and beat until the frosting is thick and fluffy. Add a teaspoon more water if it is too thick. Tint light green with the food coloring. Spoon the frosting into a resealable bag. Snip a very small corner from the bag. Pipe the celery rib lines on top of the cookies. Pipe a few leaves at the top of the cookie. Allow cookies to dry for about 30 minutes.