01 -
Preheat the oven to 350°F (180°C; 160°C fan-forced). Line two large baking sheets with parchment paper.
02 -
In a large bowl using a stand mixer with paddle attachment or handheld beater, beat together softened butter and powdered sugar until pale and creamy. Add vanilla extract and mix well.
03 -
Sift the all-purpose flour and cornstarch together. Add to the butter mixture and mix on low speed until just combined, scraping the bowl to incorporate any pockets of butter.
04 -
Lightly dust hands with flour. Using 3 teaspoons of dough per cookie, roll into balls and place on prepared baking sheets, spacing them 1 ½ inches apart. Dip a fork in flour and gently press each cookie to half its height and about 1 ½ inches wide, re-flouring the fork before each press.
05 -
Bake for 14 to 15 minutes until the edges begin to turn golden. Let cookies cool on the baking sheets for 10 minutes before transferring to a wire rack to cool completely.
06 -
Combine softened butter, powdered sugar, lemon zest, lemon juice, and optional vanilla extract in a bowl. Beat until light and creamy, scraping the sides as needed.
07 -
Spread or pipe lemon buttercream onto the flat side of half the cooled cookies, then sandwich with remaining cookies.