Step 1Heat the oven to 350 degrees. Melt the butter in a small saucepan, and set aside to cool slightly.
Step 2In a large bowl, combine the graham cracker crumbs, sugar and butter. Press the dough into each of eight (4-inch) tart shells.
Step 3Bake the crusts until set and lightly colored, about 6 minutes. Remove and set aside to cool.
Step 1Whisk together the Key lime juice, sugar and eggs in a heavy saucepan. Cook over medium heat, stirring with a wooden spoon or rubber spatula, until the mixture thickens to a custard, about 8 minutes. Be careful to scrape the sides and bottom of the pan while cooking so the mixture doesn't curdle on the edges. Remove from heat.
Step 2Strain the custard into a bowl, then cover the surface with plastic wrap and set aside to cool. When the custard comes to room temperature, add the butter and use a hand blender to blend the mixture until it is silky smooth. Divide the filling among the prepared crusts and refrigerate until set, 4 to 6 hours.
Step 1Combine the water, sugar and corn syrup in a small saucepan, then cook over medium heat until a thermometer inserted reaches 240 degrees. Meanwhile, beat the egg whites in a stand mixer or large bowl until frothy.
Step 2Remove the sugar from heat and slowly, with the mixer running, beat the sugar into the egg whites until the meringue becomes cool and holds a stiff peak.
Step 3Fill a piping bag fitted with a star tip with the meringue, and pipe the meringue, in small peaks, onto the tops of the pies.
Step 4Use a crème brûlée torch or blowtorch to brown the tips of the meringue. The pies can also be quickly placed under the broiler to brown the tips.
Step 5Garnish the tops of the pies with lime zest and serve immediately.