In a medium bowl, whisk together the flour and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together the butter, granulated sugar, and lavender on medium speed, about 2 minutes, until light and fluffy. Scrape down the sides of the bowl.
Add the egg yolk and vanilla, and mix on low speed, about 15 to 30 seconds, until just incorporated. It may not be completely combined, but we want to avoid overwhipping the eggs, which can make for uneven cookies. Once again, scrape down the sides of the bowl.
Add the flour mixture in two additions, mixing on low speed, about 30 seconds each, until just incorporated. After the second addition, scrape down the sides of the bowl, and incorporate any of the remaining flour.
On your work surface, divide the dough into 2 equal portions, and shape each into a round disk. The warmth from your hands will help the dough come together. Wrap each in plastic wrap, and refrigerate thoroughly, for at least 3 hours, or until firm. I leave it overnight, and resume the process the next day.
Preheat the oven to 350°F (180°C). Line rimmed baking sheets with parchment paper.
Unwrap one disk of dough, and roll out a ¼-inch-thick sheet. If the dough is too firm, let it sit out until it is pliable but still cold. If the dough is too soft and starts sticking while rolling, place it on a parchment-lined sheet pan, and put it back into the fridge for 5 to 10 minutes before continuing to roll it out.
Using a 2¼-inch round fluted cutter, cut out cookies from the dough. Reroll the scraps to cut additional cookies. Arrange the cookies on a sheet pan, leaving 1 inch of space between each. They shouldn’t spread in the oven.
To make the cookie tops, use a ¾-inch round cutter or piping tip, and stamp out the centers from half of the cookies. Use a toothpick to remove the dough from the piping tip.
Repeat the rolling and cutting process with the second disk of dough. You should end up with about 30 cookie tops with the centers cut out and about 30 cookie bottoms.
Bake each tray, in the middle rack of the oven, for 8 to 10 minutes, rotating the pan around halfway through baking, until the cookies are golden on the edges. Place the sheet pan on a cooling rack, and let cool for 5 minutes before transferring the cookies directly to the rack to cool completely.
To assemble, place the cookie tops on a sheet pan. Dust with confectioners’ sugar. Flip the cookie bottoms over, so they are bottom side up. Spoon 1½ teaspoons of Lemon Curd Filling in the center of each, leaving a ½-inch cookie border. Be careful not to put too much or else it will splurt out when you sandwich them. You can also fill a piping bag fitted with a small round tip, and quickly pipe the lemon curd in the center of each cookie. This is best done with cold hands to prevent the curd from warming up. Gently place the powdered cookies on top of the curd, and press slightly to sandwich them. Serve immediately.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/lavender-sandwich-cookies-lemon-curd-filling/