Take lemon juice, lemon zest and sugar in a saucepan. Turn the heat on medium.

Meantime, mix cornstarch and water in a small bowl and make a lump-free batter.

Add that to simmering lemon-sugar mixture and mix using a whisk. Within a minute or two it will thicken and becomes a thick paste.

Now add chopped butter and keep whisking until butter is melted and everything is incorporated well.

Remove it from the stove. Add condensed milk and mix.

If needed you can add yellow food color and mix.

Let it cool down completely and then store it in an airtight container.