1. In a small heavy-bottomed pot, combine whole eggs, 1 cup of the sugar, the Key lime juice and butter over medium heat. Cook, stirring constantly, until mixture reaches 175 degrees and coats the back of a spoon, about 8 minutes. Be careful not to heat mixture above that temperature or it will curdle. Pour into a bowl through a mesh strainer. Cool completely, cover with plastic wrap and refrigerate until using.
2. Heat oven to 375 degrees . Beat 1 of the egg whites. Place pie shells on a baking sheet and brush with egg white. Bake at 375 degrees for 5 to 7 minutes or until golden-brown. Cool completely on a baking rack.
3. Fill cooled piecrusts with cooled lime curd. Cover gently with plastic wrap and refrigerate for 2 hours.
4. Heat broiler to HIGH. With a hand mixer or a stand mixer fitted with whisk attachment, combine remaining 3 egg whites and the cream of tartar. Beat on high until soft peaks form. With machine running, pour in remaining 3 tbsp sugar and beat on high until stiff peaks form. Do not over-beat. Spoon meringue onto pies, using back of spoon to create texture. (Alternately, pipe meringue in a continuous spiral around curd.) Broil on HIGH for 30 seconds to 1 minute, about 7 inches from the top, until meringue is golden.