In a medium sized pot, whisk the cornstarch into the lemon juice completely while it's cold (thus no lumps) and add in the grated lemon zest as well.
Put the butter and sugar in and heat on a medium burner, stirring every few minutes to ensure nothing burns and it is mixed well.
Bring to a boil, stirring constantly and let boil for a minute.
Remove from the heat.
Take the egg yolks and whisk them until they are light yellow.
You now need to temper the eggs, which means you take some of the hot liquid from the pot and whisk it into the egg yolks completely.Whisk the yolks into the pot of hot liquid until incorporated thoroughly.
Return to the stove, cooking for another minute or two, whisking the entire time.
Cool for an hour or so until it's set and thick, ready to be used between cake layers.
Notes
This is also good on toast, just like a lemon curd is!This recipe yields around 1 1/2 cups of filling ,enough for a layer cake.