For the Cookies:

1. Whiz up 2/3 cup of toasted pistachios in the food processor until finely ground, about 8-10 pulses should do it. In a small mixing bowl, whisk together flour, sea salt, and ground pistachios.
2. In a large mixing bowl beat the butter and granulated sugar on medium until light and fluffy, scraping down the sides of the bowl as needed, about 3 minutes. Add the egg and vanilla mix until fully combined, about a minute.
3. With your mixer on low, slowly add in the flour/pistachio mix, in two or three additions. Once all the flour mixture is added, increase the speed to medium and mix until just combined. Turn out onto a lightly floured surface and split the dough in two. Press each half into a disk, about one inch thick and wrap each tightly with plastic wrap. Refrigerate both disks for at least one hour and up to one day.
4. When you're ready to back the cookies, preheat the oven to 375* and take the dough out of the fridge and let it sit at room temperature for about 10 minutes (the longer you've left it in the fridge, the longer you'll probably need to let it sit on the counter) so that it's easier to work with. Meanwhile, line two rimmed baking sheets with parchment paper. Lightly flour your work surface and with a rolling pin, roll the dough to about 1/8" thickness. Using a floured 2 1/2 inch round cutter, cut out the cookies and place on baking sheet. Gently press the leftover scraps of dough together and re-roll the dough and cut out more cookies.*
5. Bake in the oven for 7-9 minutes until the cookies are set around the edges and lightly golden, rotating once during baking. Remove from oven and allow to cool for five minutes on the baking sheet, then transfer to cooling rack and allow to cool completely. Repeat the baking process with the second tray of cookies.

For the Lemon Buttercream Filling:

1. Beat together the butter and powdered sugar in a medium mixing bowl until light and fluffy. Add in the lemon zest, lemon juice and salt. Mix until combined.

For the Icing:

1. In a small mixing bowl, stir together the powdered sugar and lemon juice. This should be enough juice. It will look dry at first and like you need to add more, but resisted this temptation! Just keep stirring. You want it to be thin enough to drip from the spoon so that you can easily decorate the cookies, but thick enough so that it doesn't just slid off the cookies.

Assembly:

1. Once completely cooled, spread (or use a piping bag) half the cookies with about 1 tablespoon of the buttercream. Top with a second cookie. Drizzle one side of each sandwich cookie with the lemon icing topping and sprinkle the chopped toasted pistachios. Allow the icing to set completely and enjoy!

*I bake one sheet of cookies at a time so that they bake as evenly as possible. You can go ahead and fill the second baking sheet and just put it in the fridge to keep cool until the first batch comes out of the oven.