Directions

1. Preheat the oven to 300° F with a rack in the middle position. Line an 8-inch square baking pan with parchment paper so that the edges come up over two sides. Set aside.
2. Make the cookies  Put the butter, both sugars, and salt into a large mixing bowl. Use a hand mixer to cream the ingredients until they are fluffy and the consistency of butter cream frosting, about 4 minutes (To avoid creating clouds of powdered sugar, start the mixer on low speed and increase to high after the sugar is incorporated.)
3. Add the lavender and lemon zest to the butter mixture and beat for an additional minute. Add the flour and beat on low until it’s no longer visible and the mixture is crumbly, like coarse sand. The ‘dough’ will not come together in a ball but should stick together when you squeeze a small handful in your palm.
4. Transfer the crumbly dough to the prepared baking dish. Use your hands to press it evenly into the pan. If desired, use a large offset spatula to smooth and flatten the top. Use a fork or toothpick to lightly prick small holes across the surface.
5. Bake for 20 minutes, then remove the pan from the oven. Use a sharp paring knife to carefully score the cookie block into bars (3 rows, 8 columns, cut about half-depth). Return the pan to the oven and bake until the edges are lightly golden, 25-35 additional minutes (check every 10 minutes). Remove the pan from the oven and let the cookies cool completely.
6. Use the overhanging parchment as handles to lift the block of cookies onto a cutting board. Use a large, sharp knife to cut the bars apart at the scored lines. Transfer the bars to a metal rack set over a baking sheet or to a clean sheet of baking parchment.
7. Make the glaze and decorate the cookies  Put powdered sugar and lemon juice into a medium bowl and stir until smooth. Use a spoon to drizzle thin ribbons of glaze over the cookies. While the glaze is wet, sprinkle the cookies with the lemon zest and chopped lavender. Let dry for about an hour before serving; longer before packing into a container.