Whisk the eggs, egg yolks, sugar and key lime juice together in a heavy-bottomed stainless-steel pan. Cook over medium low heat, stirring continuously, alternating between a whisk and rubber spatula, until the key lime curd has thickened to the consistency of pastry cream and coats the back of the spatula. Make sure you are continuously stirring the curd to prevent scrambling the eggs.
Remove the key lime curd from the heat. Add the butter a little at a time, whisking continuously. Once the butter is completely melted and incorporated into the curd, add a little more butter until all the butter has been added to the curd. Season with a pinch of salt. Strain the key lime curd to make sure it is very smooth.
Pour the key lime curd into the tart shells. Chill the tarts in the refrigerator until the lime curd has completely cooled. It will become thicker as it cools. Decorate tarts with whip cream, meringue or fresh fruit. Enjoy!