1. Preheat oven to 350 degrees. Line a baking sheet with a Silpat baking mat or parchment. Spray lightly with nonstick cooking spray, and set aside.

2. In a small heavy-bottomed saucepan, combine cream, lemon zest, and thyme sprigs, and bring to a simmer over low heat. Remove saucepan from heat, and allow the flavors to infuse the cream, 15 to 20 minutes. Pour cream through a sieve, and reserve.

3. In a large bowl, whisk together egg whites and sugar until well combined. Whisk in flour and salt. Add melted butter, cream mixture, and lemon extract, and whisk until well combined, about 30 seconds.

4. Pour batter into a medium pastry bag fitted with a #11 round tip. Pipe silver-dollar-size (about 1 inch across) circles onto prepared baking sheet, leaving 2 inches between each cookie. Sprinkle each cookie with the picked thyme leaves, and bake for 15 minutes, or until edges turn brown. Transfer cookies to a wire rack to cool.