Heat a pot over medium heat. With a wooden spoon, mix together the sugar and cornstarch until fully combined. Add the butter and lemon juice and whisk until combined. Bring to a simmer and then reduce to medium heat.
Add the egg yolks to the saucepan. Whisking every 30 seconds or so,, cook for 15-20 minutes. It should thicken and coat the back of a spoon. Don’t worry if you see some egg yolk bits in there. We will strain them! Transfer the curd through a fine mesh strainer into a large shallow bowl (preferably not metal) and allow it to cool down for an hour. It thickens as it cools down, and you'll notice a gelatinous-ish texture.
Store in a sealed container for up to a month in the fridge.