For the cookies

1. Preheat the oven to 350°F/177°C.   Line two large baking sheet pans with parchment paper or silicone baking mats.  Set aside.
2. In a large bowl using a hand-held mixer or stand mixer with the paddle attachment, beat the butter on medium speed until completely smooth and creamy.  Add the sugar and mix on medium speed until fluffy and light in color.  Mix in the egg and scrape down the sides and bottom of the bowl.  Mix in the vanilla and lemon zest.
3. In a medium bowl, sift together the flour, baking powder, and salt.  On low speed, slowly add the flour mixture into the butter mixture until combined.
4. On a floured surface, roll the dough to a 1/4-inch thickness.  Using a floured 2-inch fluted biscuit cutter, cut out cookies. Re-roll scraps and continue to cut cookies with the remaining dough. Place cookies on prepared baking sheets, allowing about 1.5-inches between each cookie.
5. Bake for 8-10 minutes or until the bottom edges are very lightly browned.  Allow the cookies to cool on the baking sheet pans for 5 minutes before transferring to a wire rack to cool completely.

For the filling

1. In a large bowl using a hand-held mixer or stand mixer fitted with the paddle attachment, beat the butter on medium-high speed for 5 minutes.  With the mixer on low speed, slowly add in the powdered sugar and salt, beating until the sugar is completely incorporated.  Add the vanilla extract and lemon zest and beat until combined.  Add, one tablespoon at a time, the lemon juice until desired consistency is achieved.  Beat the buttercream on medium-high for an additional 3-4 minutes until smooth and creamy.

To assemble sandwich cookies

1. Fill a pastry bag with the buttercream and cut 1/8 to 1/4-inch off the tip.  Pipe approximately 1 tablespoon of the filling on half of the cookies.  Top with remaining cookies and press gently.