Process or blend while adding the butter cubes piece by piece, making sure each piece is incorporated before adding the next.
When all of the butter is incorporated, add the lemon zest.
Pour the lemon curd into the cooled tart shell, using a spatula to smooth it.
Refrigerate the tart overnight, or at least 8 hours.
Watch your tart shell carefully once it's been in the oven for 10 minutes. Oven temperatures vary, and the tart shell can easily burn or become too dark.Use a candy thermometer to measure the temperature of your lemon curd while it cooks.Be careful not to boil the curd, simply heat it until it thickens, which should happen well before the boiling point.If you use a blender to incorporate the butter into the filling, make sure that it has a vented lid in order to prevent steam build up and the filling erupting.Don't skip the chilling time. It's important to allow the filling to set so that it slices perfectly. Refrigerate the tart overnight for best results.