In a saucepan, pour about 2 inches of water and bring it to a simmer. Place a bowl on top without touching the water.
In the bowl, combine lime juice, lemon juice, eggs, yolks, sugar, and salt. Whisk continuously over the simmering water for 10-12 minutes or until thick (180°F on a thermometer).
Remove from heat and cool to 140°F, stirring occasionally.
Whisk in butter, one tablespoon at a time, until melted and smooth. Stir in the lime zest.
Pour the slightly warm curd over the cooled cheesecake. Refrigerate for at least 2 hours or overnight until set.