In a medium saucepan combine water, sugar, cornstarch and salt. Bring to a simmer over medium heat, whisking constantly.
When the mixture turns translucent, about 5 minutes, add egg yolks, 2 at a time.
Whisk in lemon zest, lemon juice and butter.
Let mixture come back to a simmer then remove from heat. Pour into a bowl and cover warm curd with wax paper to prevent a skin from forming on top while it cools.
Serve straight up or with fresh berries when you have some.
Notes
Lemon curd will keep in the fridge for 2 weeks but that's never happened at my house.