Grab a saucepan, and add all the ingredients, expect for the butter inside.
Place this in medium-low heat. Use a rubber spatula to stir the whole time. The goal is to break any egg bits or cornstarch, so the curd can be smooth.
You can use a whisker if it helps you more, just until everything's broken and you cannot see any bits, then switch to a rubber spatula.
Once the curd start to thicken out, take it off the heat and add the butter, one by one, while whisking. Stir until the butter has fully melted and you have a runny, silky curd.
Now you can transfer the curd into a plate or heatproof jar and allow it to come in room temperature.
Once it does, place this on the fridge, until you need to use it and enjoy!