In a food processor, pulse together the flour, powdered sugar, cornstarch, salt, lemon zest, and lavender until well blended. Add butter over the mixture and pulse until the mixture resemnles wet sand.
In a small bowl, whisk together the egg yolks and extracts. Drizzle over the dough and pulse only until large clumps of dough form.
Pour the dough onto a work surgface and cut in half. Form each piece into a log that is boat 8 inches long and 2 inches in diameter. Wrap each log in plastic wrap and twist the ends closed to smooth the surgace of the logs.
Refrigerate until the dough is chilled completely. This will take at least 4 hours, I keep mine in the refrigerator overnight.
When ready to bake cookies, preheat oven to 350F degrees. Line two baking sheets with parchment paper.
Use a sharp knife, cut each log into 18 rounds. Each about 1/2 inch thick. Rotate the logs a quarter-turn between cuts to keep the logs round. Arrange the cookies at least 1 inch apart on the prepared baking sheets.
Bake for 10-13 minutes or until firm and golden on edges.