Bring a small pot of water to a gentle simmer. Nest a heatproof bowl over it.
Whisk lemon juice, sugar, yolks, zest, and a small pinch of salt until smooth.
Cook over gentle steam, stirring constantly and scraping the sides and bottom, until the curd reaches 170 F and coats the back of a spoon.
Off heat, whisk in butter until glossy.
Strain through a fine mesh sieve into a clean container. Press parchment onto the surface. Cool, then refrigerate until fully cold.
Swirl into churned ice cream at pack time. Layer in spoonfuls and make one or two gentle figure eights.
