Preheat oven to 325°. Line several rimmed baking sheets with parchment paper.
In a large mixing bowl, beat together butter and sugar with a mixer at high speed until light and fluffy, approximately 5 minutes. Add egg, lemon zest, lemon extract, and vanilla extract, beating until incorporated.
In a medium bowl, whisk together flour, baking powder, and salt until combined. Gradually add flour mixture to butter mixture, beating until dough comes together.
Place half of dough between 2 sheets of parchment paper or wax paper. Using a rolling pin, roll dough to a scant ¼-inch thickness. Transfer dough and parchment paper to another rimmed baking sheet, and freeze for 15 minutes. Repeat with remaining dough half.
Remove dough from freezer, and remove parchment paper from dough. Using a 1½-inch fluted round cutter dipped in flour, cut 76 rounds from chilled dough. Place rounds 2 inches apart on prepared baking sheets.
Bake until edges of cookies are very light golden brown, 8 to 10 minutes. Transfer cookies to wire racks, and let cool completely. Store in an airtight container with layers separated by wax paper until ready to assemble sandwich cookies.
Place Elderflower Buttercream in a piping bag fitted with an open-star tip (Wilton #32). Working in a clockwise circle, pipe buttercream onto flat bottoms of 38 cookies. Gently press a remaining plain cookie on top of each, flat sides down. Serve immediately or store in an airtight container with layers separated by wax paper, and refrigerate for up to 2 days.
Recipe by TeaTime Magazine at https://teatimemagazine.com/lemon-elderflower-sandwich-cookies/