In a small saucepan, warm the butter over medium-low heat until melted. Remove from heat and allow to cool to lukewarm. Whisk in the egg yolks and set aside.

In a medium bowl, add the lemon juice, zest, sugar and egg whites. Whisk to combine. Pass through a fine-meshed sieve into the butter and yolk mixture, press the zest solids to extract as much liquid as possible. Return the pot to the stove and cook over medium heat, stirring constantly, until the mixture coats the back of a spoon and is glossy. Pour into a glass jar and place the jar in an ice bath for 20 minutes. Cap tightly and store in the refrigerator for up to 1 week.