To make the lemon curd: Bring a pot of water to a simmer over medium-low heat. Combine the egg yolks, sugar, lemon juice, and zest in a metal or glass heat-resistant bowl and whisk until smooth. Set the bowl over the simmering water, without letting the bottom touch, and continue to whisk. Keep working-out that arm and whisk it vigorously for a good 10 minutes, until the curd has doubled in volume and is very thick and yellow. Don't let it boil. Remove the bowl from heat and whisk in the butter, a couple of chunks at a time, until melted. Refrigerate until the custard is cold and firm.