Heat oven to 350 degrees F. Line a baking sheet with a silpat. Spray lightly with nonstick cooking spray and set aside.
In a small, heavy-bottomed saucepan, combine the 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
In a large bowl, whisk together egg whites and sugar until well combined. Whisk in flour and salt. Add melted butter, cream, and lemon extract and whisk until well combined, about 30 seconds.
Pour batter into a medium pastry bag fitted with a #11 round tip. Pipe silver dollar-sized (about 1-inch across) circles onto prepared baking sheet, leaving 2 inches between each cookie. Sprinkle each cookie with the picked thyme and bake for 15 minutes, or until edges turn brown. Transfer cookies to a wire rack to cool.
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