6 tablespoons unsalted butter, cut into pieces and softened
In a food processor, combine the sugar with the lemon zest and lime zest. Pulse 10 times until the sugar becomes light green.
In a medium heatproof bowl, whisk the eggs with the egg yolks, citrus sugar and orange and lime juices. Set the bowl over but not in a pan of simmering water and cook, stirring constantly with a wooden spoon, until thickened, about 10 minutes.
Remove the bowl from the heat and whisk in the butter, a few pieces at a time, until fully incorporated. Press the curd through a fine strainer set over a small bowl to remove any zest or scrambled egg. Press a piece of plastic wrap directly on the curd and refrigerate until chilled, at least 1 hour.
The lemon curd can be refrigerated for up to 1 day.