Make the Crust: In a mixing bowl, combine flour and powdered sugar. Cut in the cubed butter until the mixture resembles coarse crumbs. Stir in the egg yolk and add cold water, one tablespoon at a time, until a dough forms.
Chill the Dough: Shape the dough into a ball, wrap in plastic wrap, and refrigerate for at least 30 minutes.
Preheat Oven: Preheat your oven to 350°F (175°C).
Roll and Cut: On a floured surface, roll out the chilled dough to about 1/8-inch thickness. Cut into circles to fit a mini muffin tin, and press the dough into the cups.
Bake the Shells: Prick each crust with a fork and bake for 10-12 minutes, or until lightly golden. Remove and let cool.
Prepare Lemon Filling: In a saucepan, whisk together the granulated sugar, cornstarch, and water. Cook over medium heat until thickened, stirring constantly. Remove from heat.
Add Eggs and Lemon: In a separate bowl, whisk the egg yolks. Slowly add a bit of the hot mixture to the yolks to temper them, then whisk it all back into the saucepan. Stir in lemon juice, zest, and butter until smooth.
Fill Pie Shells: Spoon the lemon filling into the cooled pie crusts, filling each about 3/4 full.
Make Meringue: In a mixing bowl, beat the egg whites and cream of tartar until soft peaks form. Gradually add the sugar, continuing to beat until stiff peaks form.
Top Pies: Spoon the meringue over the lemon filling, making sure to seal the edges to prevent shrinking. Use the back of a spoon to create peaks and swirls in the meringue.
Bake Again: Return to the oven and bake for an additional 10-12 minutes, or until the meringue is golden brown.
Cool and Serve: Allow the mini pies to cool completely before serving to let the filling set fully.
Notes
Arrange the mini pies on a serving plate and garnish with fresh lemon zest or a sprig of mint for a pop of color.