Add the granulated sugar and lemon zest to the bowl of a stand mixer. Rub the zest into the sugar with your fingers for 30–60 seconds until very fragrant. This helps release the natural oils and enhances the lemon flavor.

Add the unsalted butter to the lemon sugar. Beat with the paddle attachment on medium speed until light and fluffy, about 3 minutes.

Add the sea salt, egg, and vanilla extract and beat on medium speed until combined, about 1 minute. Add the fresh lemon juice and mix until just combined. The mixture may look slightly curdled, but this is normal—it will smooth out once the flour is added.

With the mixer running on low speed, slowly add the all-purpose flour in batches. Continue mixing until the dough is just combined and begins to ball up around the paddle. The dough will be soft, but it should hold together easily when pressed. Avoid overmixing.

Lightly flour your work surface and rolling pin. Turn the dough out and roll it to about 1/8-inch thick. Use a 2-inch round cookie cutter to cut out the lemon cookies.

Transfer the cookies to the prepared baking sheets, leaving about 1/2 inch between each one. Sprinkle a pinch of granulated sugar over the top of each cookie. Bake for 10 minutes, or until the cookies are just set. The edges should feel firm, but the cookies will remain pale with little to no browning. Let the cookies cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely. Bake one sheet at a time, placing the first sheet in the oven while the second. Once the first sheet is done, bake the second.

Once the cookies are completely cool, beat the unsalted butter and sea salt in a stand mixer with the paddle attachment for 1–2 minutes until creamy. With the mixer on low speed, gradually add the powdered sugar 1/2 cup at a time. Stop and scrape down the bowl, then mix on medium speed for 1 minute until light and fluffy. Add the lemon curd and lemon zest and mix on medium speed for 1–2 minutes until smooth and fully combined.

Flip over half of the cookies so the flat side is facing up. Spread or pipe the lemon buttercream filling onto that half. Top with the remaining cookies, gently pressing the bottoms of the plain cookies onto the filling to create each sandwich cookie.