In a medium metal bowl, whisk together egg, egg yolks, sugar, and lime juice. Set bowl over a saucepan of simmering water (bottom of bowl should not touch water). Whisk constantly until an instant-read thermometer registers 180°, approximately 6 minutes. Remove from heat. Stir in butter, whisking until smooth.

Pour curd evenly into 6 crusts. Press plastic wrap directly onto surface of curd, covering completely. Place tarts on a rimmed baking sheet, and refrigerate until chilled, about 3 hours.

Garnish with fresh blackberries and lime zest curls, if desired.