Prep:Heat oven to 350°F. Line a 9x13 inch pan with parchment paper.
Crust:Mix butter, sugar, vanilla and salt in a medium bowl. Stir in flour, then press into the prepared pan. Bake around 15 minutes in the hot oven.
Filling:Whisk together sugar, eggs, lemon juice and lemon zest in a medium bowl until smooth. Whisk in flour and cornstarch until smooth.
Finish bars:Pour the filling over the pre-baked crust. Bake around 20-25 minutes, until the bars are just set.
Cool:Let bars cool to room temperature, then place in the fridge after about 2 hours. Decorate with icing sugar and slice to serve.
Video
Notes
Baking tips
Don't over-mix the shortbread base once you've added the flour - only mix until the flour is combined. Otherwise, the base will turn out gummy instead of buttery and crumbly like a cookie.
Mix the lemon filling just before pouring it over the hot crust, otherwise the eggs could start curdling if it sits on the counter too long.
Do not overtake the bars, they should still have a bit of a jiggle when you take them out.
Cool the bars well before slicing them, otherwise they're still too creamy and won't come out as neat squares.
Ingredient substitutions
Butter: You can use margarine in place of the butter for the base. The taste and texture will be a little different versus using butter.
Lemon juice: Bottled or fresh both work; fresh tends to curdle the eggs a little faster though, so do make sure to get the filling in the oven as soon as it's mixed.
Lemon zest: Feel free to skip if you don't have any on hand. The zest does add an extra lemony kick, but the bars are still plenty tart without it.
Other citrus: You can also use another citrus juice like orange or lime for a different flavor if you want.
Cornstarch: If you don't have any cornstarch, use 2 more tablespoons of flour instead. The filling will change texture a little, but it's still delicious.
Storage
The bars keep well covered in the fridge for up to 3 days. The base tends to soften in the fridge, but they're still delicious to eat.