01 -
Set your oven to 350°F and line your trays with baking paper.
02 -
Whip together the butter, vanilla, and sugar until it's light and fluffy. Gradually mix in the flour and cornstarch, adding them in three parts.
03 -
Scoop out dough in tablespoon amounts, roll into balls, then flatten gently with a fork to about 1cm thick.
04 -
Pop them in the oven for 15 minutes. Swap the trays midway through baking, then let them cool entirely on a rack.
05 -
Whip the butter until it's creamy, then gradually blend in the powdered sugar. Add lemon zest and juice at the end.
06 -
Pipe a swirl of filling onto one cookie, then sandwich another cookie on top. Let them chill in the fridge for an hour before serving.