Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 2-3 minutes with an electric mixer.

Beat in the egg, lemon juice, and lemon zest until well combined.

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt.

Gradually add the dry ingredient mixture to the wet ingredients, stirring until just combined. Fold in the sourdough discard until fully incorporated.

Using a cookie scoop or tablespoon, drop the cookie dough onto the prepared baking sheet, leaving space between each cookie.

Bake for 10-12 minutes, until the edges are lightly golden.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Once cooled, spread a generous amount of buttercream or lemon curd on the flat side of one cookie and top with another cookie to make a sandwich.

Repeat until all cookies are filled.