1. Combine yolks, sugar, and lemon zest and juice in a medium heavy-bottom saucepan. Cook over medium-low heat, stirring constantly with a wooden spoon, until mixture is thick enough to coat the back of the spoon, 8 to 10 minutes.

2. Pass mixture through a fine sieve into a medium bowl. Add butter, one piece at a time, stirring after each until butter is melted and combined.

3. Cover with plastic wrap, pressing directly onto the surface to prevent a skin from forming. Refrigerate until firm and thoroughly chilled, at least 1 hour.