Beat the egg yolks in bowl.
Heat lemon juice, sugar and lemon zest in a saucepan over medium-low until warm.
Pour ¼ of the lemon juice mixture into the egg yolks and whisk to temper the eggs. Add another quarter of the lemon mixture to the egg yolks and whisk.
Combine the remaining lemon mixture and yolks in a saucepan and heat over medium-low heat.
Add one piece of butter at a time, whisking until melted. Continue adding the butter, one piece at a time, until it has all been incorporated.
Continue whisking until the whisk leaves lines across the mixture, about 5 minutes.
OPTIONAL: Strain through a mesh strainer or sieve to smooth out lumps and remove any pieces of zest.
Pour in a bowl, cover tightly with plastic wrap, and let cool on the counter for at least an hour.
After the mixture has cooled, place in a airtight container and store in the fridge for 1-2 weeks.