Step 1

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.

Step 2

Let the mix simmer for 1 minute.

Step 3

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.

Step 4

Take it off the heat and stir the condensed milk in. That's it.

Step 5

Transfer to a sterilised jar, bring to room temperature, then store in the fridge for up to a week.