line a 9 x 13 inch pan with parchment and lightly grease
mix flour powdered sugar and salt then cut in cold butter until crumbly
press firmly into pan in an even layer
bake until lightly golden at edges 18 to 20 minutes.
Custard filling and bake
Whisk eggs and sugar until smooth
whisk in flour and cornstarch until no dry spots
whisk in lemon juice lemon zest salt heavy cream and vanilla until fully combined
pour over hot crust and return to oven
bake until center is set with a slight jiggle 20 to 24 minutes.
Chill and serve
Cool to room temperature then chill at least 2 hours for clean slices
lift out and cut into bars
dust with powdered sugar just before serving.
Notes
1. Use fresh lemon juice for best flavor 2. chill thoroughly before slicing and wipe knife between cuts 3. substitute lime juice and zest for lime bars 4. for extra firm filling add 1 tbsp cornstarch 5. store covered in the refrigerator up to 5 days