Use pastillage just after kneading.
- 5 cups plus 2 1/2 Tablespoons confectioners' sugar, plus more if needed
- 6 tablespoons cornstarch, plus more for dusting
- 2 teaspoons powdered gelatin
- 1 large egg white
- Juice of 1/2 lemon
Sift confectioners' sugar and cornstarch into bowl of an electric mixer. Set aside.
In a small bowl, sprinkle gelatin over 2 tablespoons cold water. Let stand 5 minutes.
In a medium heatproof bowl, combine egg white and 1 tablespoon water. Set bowl over a pot of gently simmering water; stir continuously until warm to the touch, about 2 minutes. Remove from heat; add gelatin mixture. Stir until dissolved. If gelatin does not dissolve completely, return mixture to double boiler; stir until completely dissolved.
Remove from heat, and immediately add to cornstarch mixture. Add lemon juice; mix on low until mixture comes together. Do not overmix. If it appears too dry, add a few drops of lukewarm water, if it appears too wet, sprinkle in confectioners' sugar that has been sifted.
Transfer pastillage to a clean work surface dusted lightly with cornstarch. Knead until smooth, 2 to 3 times. Using a pastry brush, dust off excess cornstarch. Form into a ball, and wrap in plastic wrap. Chill up to 24 hours (bring to room temperature before rolling).
To roll, take a small piece of pastillage from the ball; place on a clean work surface dusted with cornstarch (keep the rest wrapped). Roll small piece out to 4 1/2-by-5 1/2-inches and 3/16 inch thick. Cut lengthwise into eight 1/2-inch strips.
For top of fence, cut a half-moon along top edge of pickets; use the edge of a 10-inch bowl as a guide. The longest pickets (on both ends) should be 5 1/2 inches tall, the shortest (in middle) 3 3/4 inches. With a spatula, transfer pickets to a parchment-lined baking sheet to dry at least 6 hours.
Repeat steps 6 and 7 four times.
For posts, roll out a 3/16-inch-thick piece of pastillage. With a paring knife cut into a 3-by-6-inch rectangle. Cut rectangle lengthwise into five 1/2-inch-wide strips. Cut top of each post to a point.
Transfer posts to a parchment-lined baking sheet to dry, at least 6 hours. Immediately after icing cake, apply pickets and posts 1/2 inch apart around perimeter.