Prepare dry ingredients: In a medium mixing bowl, whisk together the all-purpose flour and salt until fully combined. Set aside.

Cream butter and sugar: Using an electric mixer fitted with the paddle attachment, cream together room temperature unsalted butter and superfine sugar on medium speed for 3-4 minutes until light and fluffy. Scrape down the bowl with a rubber spatula as needed.

Add lemon flavorings: Mix in lemon zest, fresh lemon juice, and lemon extract until fully incorporated. The mixture may look slightly curdled - this is normal.

Incorporate dry ingredients: With mixer on low speed, gradually add the flour mixture in three additions, mixing just until combined after each. The dough should come together but still be slightly sticky.

Shape and chill: Turn dough out onto a lightly floured surface and gently form into a flat disk. Wrap tightly in plastic wrap and refrigerate for at least 4 hours (or up to 3 days). This chilling step is crucial for easy rolling and prevents spreading during baking.

Rolling and Cutting:

Prepare work surface: When ready to bake, remove dough from fridge and let sit at room temperature for 10 minutes. Lightly flour your work surface and rolling pin.

Roll out dough: Roll dough to 1/4-inch thickness, rotating and flouring as needed to prevent sticking. For even thickness, use rolling pin guides or place two 1/4-inch thick dowels on either side of the dough.

Cut cookies: Using a 2-inch round cookie cutter, cut out as many cookies as possible. Gather scraps, re-roll once (no more or cookies may become tough), and cut additional cookies.

Prep baking sheets: Line cookie sheets with parchment paper. Place cut cookies about 1 inch apart - they won't spread much but need air circulation.

Chill before baking: Place baking sheets with cut cookies in the refrigerator for 15 minutes while preheating oven to 375°F (190°C). This helps maintain shape.

Bake: Bake one sheet at a time in center rack for 10-12 minutes until edges are just beginning to turn golden. Rotate pan halfway through for even baking.

Cool: Let cookies cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely. They'll firm up as they cool.

Prepare buttercream: In clean mixer bowl, beat room temperature butter until smooth. Gradually add powdered sugar, then lemon juice and zest, beating until light and fluffy (about 3 minutes).

Assembling:

Pair cookies: Match cookies by size. Flip half of them upside down (these will be the bottoms).

Fill cookies: Pipe or spread about 1 teaspoon filling onto each bottom cookie. Gently press matching top cookie to create sandwich.

Final set: Let assembled cookies sit for 30 minutes before serving to allow filling to set slightly.