Beat the sugar and butter with an electric mixer in a large bowl until slightly lighter in colour, a couple of minutes will do.
Add the eggs and yolks, one at a time, beating for 30 seconds in between.
Transfer this into a medium sized saucepan.
Get a wooden spoon and add the lemon juice in stirring continuously. Don't worry about the change in texture. Add the lemon zest now, if using.
Place on a very low heat and keep stirring with the spoon until the mixture becomes smooth and thickens, about 10-15 minutes. You should feel some resistance against the spoon. The mixture will still be a bit runny, it'll thicken up as it cools.
Transfer to sterilized jars and keep in the fridge up to 2 weeks. Freezes well.