Melt the butter, sugar and lemon zest in a saucepot over medium heat until the butter melts fully (the sugar will not dissolve at this point). Whisk the eggs lemon juice and cornstarch together and whisk this into the pot. Continue to whisk the curd over medium heat until it thickens and just begins to bubble, about eight minutes. Pour this through a strainer and cover the surface of the curd with plastic wrap. Cool the curd to room temperature, then chill completely.
The curd will keep, refrigerated, for up to three weeks.