Add the sugar and zest to the bowl of a food processor. Pulse the sugar and zest until moistened and the zest has broken down until smaller bits. Transfer to a bowl.

Add the egg yolks to the sugar mixture and whisk until smooth, approximately 1 minute.

Whisk in the lemon juice (you should have 1/3 of a cup, if not...add a bit more lemon juice).

Fill a medium saucepan with enough water to measure 1 inch up the side of the pan. Bring the water to a simmer.

Place a heat proof bowl over the saucepan (bowl should not be touching the water) and pour the lemon mixture into the bowl.

Whisk the mixture until thickened and can coat the back of a spoon, approximately 8-10 minutes.

Immediately remove the bowl from the heat and stir in the butter, two cubes at a time. Stir the butter until completely melted before adding more.

Transfer to an airtight container and store in the refrigerator for up to 2 weeks.