Place the butter, sugar, lemon juice and lemon zest (if using) in a small saucepan on medium heat and bring to a gentle simmer. Stir, as the butter is melting, to mix.
100 g salted butter, 100 g white sugar, 200 ml lemon juice , zest of 2 lemons
Let the mix simmer for 1 minute.
Mix the cornflour and water to a smooth paste, and very slowly drizzle this into the lemon and sugar mix, stirring constantly, until the mixture has thickened.
2 Tbsp cornflour, 2 Tbsp cool water
Take it off the heat and stir the condensed milk in. That's it.
100 g condensed milk
Transfer to a sterilised jar, bring to room temperature, then store in the fridge for up to a week.