Combine water, cornstarch, sugar, salt & egg yolks in a saucepan over medium heat.
Stir vigorously to mix.
After filling thickens, whisk in lemon zest & juice. Boil for 1 minute.
Remove from heat & stir in butter. Allow to cool. Pour through a mesh sieve.
Transfer to a container & cover with plastic. Refrigerate to cool completely.
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