Start by preheating your oven to 350°F (175°C). Line two baking sheets with parchment paper. This serves two purposes: it prevents sticking and makes cleanup a breeze.

In a large bowl, cream together the softened butter and confectioners’ sugar. Use a hand mixer or a stand mixer to get a smooth, fluffy mixture. You want it to be airy—this is what creates the tender texture of the cookies. Add the flour, salt, pistachios, vanilla extract, and lemon zest. Mix gently until the dough comes together. Avoid overmixing; we want our cookies to have a delicate texture.

Using a cookie scoop or your hands, form round balls of dough (about 1 inch in size) and place them on the prepared baking sheets. Leave some space between each ball; they will spread out a bit while baking. Once they are all on the sheet, use your thumb (or the back of a measuring spoon) to press an indentation into each cookie. This is the "thumbprint" part of the process—and it’s also where the fun begins!

Pop those cookie sheets into the preheated oven. Bake for about 10-12 minutes or until the edges are lightly golden. Keep an eye on them so they don’t overbake; we still want that soft center.

Once baked, remove the cookies from the oven and let them cool on the baking sheets for a few minutes before transferring them to wire racks to cool completely. Once cooled, fill each thumbprint with your desired filling—be it nut butter, jam, or that luxurious pistachio cream—and watch your cookies transform!

For a fancy finish, melt some chocolate in a microwave-safe container. Drizzle it over the filled cookies for that extra touch. Allow the chocolate to set before serving.