In a small-to-medium saucepan, combine the lemon juice with half the granulated sugar. Set on a burner on medium–high heat.
Meanwhile in a medium heatproof bowl, whisk together the egg yolks with the rest of the sugar. Whisk vigorously until the mixture turns a very pale, light yellow. Set aside.
Place another bowl with a strainer over top next to the stove, and have the chopped butter ready on a plate close by as well. Have a heatproof spatula ready too. Do all this before going any further.
When the lemon juice just comes to a boil, take the pot off the burner and pour the juice over the sugary egg mixture. Whisk to temper the eggs and combine all the ingredients together. Pour the mixture back into the saucepan and return the pot back onto the burner. Whisk the mixture over medium–high heat until it is thick enough to coat the back of a spoon and cling to it. You can use a thermometer to monitor the temperature: a properly cooked lemon curd is cooked to 77–82 ºC or 170–180ºF.
After the right set and temperature are reached, immediately take the pot off the burner and turn it off. Drop the butter into the saucepan, a few pieces at a time, whisking constantly and when all the butter has disappeared into the curd, transfer it all to the strainer set over a bowl. Collect the curd in the bowl, then cover with cling film pressed directly on the surface to protect it from the air. Chill overnight.
For the brightest, best flavour, please use freshly squeezed lemon juice from fresh lemons
Do not substitute the lemon juice sold in bottles at the grocery store! It won't yield the same flavour or colour.