In a medium saucepan, heat milk over medium-high heat, whisking occasionally, until steaming (do not boil). Remove from heat.
In a medium bowl, whisk together granulated sugar, egg yolk, cornstarch, zest, and salt. Gradually add ¼ cup hot milk to egg mixture, whisking constantly. Return mixture to saucepan, whisking to combine. Cook over medium heat, whisking constantly, until boiling. Let cook for 1 minute to remove cornstarch flavor. Remove from heat; whisk in butter until melted and combined. Whisk in lime juice. Pour into a medium bowl and cover with plastic wrap, pressing wrap directly onto surface to prevent a film from forming. Refrigerate for at least 1 hour before using.
Preheat oven to 400°. Line a baking sheet with parchment paper.
Unroll each piecrust onto a lightly floured surface. Using a pastry wheel or sharp knife, cut 4 (4-inch) squares from each piecrust. Place squares on prepared pan. Spoon a heaping tablespoon of lime filling into center of each square. In a small bowl, whisk together egg and 1 tablespoon water. Brush edges of dough with egg wash. Fold dough in half over filling to form a triangle. Using the tines of a fork dipped in flour, crimp edges to seal each pie. Refrigerate for 10 minutes. Using a sharp knife, cut 2 small slits into top of each pie to vent.
Bake until golden brown, 15 to 20 minutes. Let cool completely on wire rack.
In a small bowl, whisk together confectioners’ sugar and cream to desired consistency. Drizzle over pies. Garnish with lime zest, if desired.
Notes
KITCHEN TIP-If you like cold Key lime pie, pop these into the refrigerator before serving.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/key-lime-hand-pies/