In a mixing bowl, combine flour, powdered sugar, and salt. Cut in the chilled butter until the mixture resembles coarse crumbs. Stir in the egg yolk and enough cold water to form a dough. Roll out the dough and cut into circles to fit mini tart pans. Prick the bases with a fork and chill in the refrigerator for 30 minutes.

Preheat the oven to 175°C (350°F). Bake the tart shells for 15-20 minutes or until golden. Let them cool completely.

In a saucepan, whisk together lemon juice, sugar, eggs, lemon zest, and vanilla. Cook over medium heat until thickened, stirring constantly. Remove from heat and let cool slightly before pouring into the cooled tart shells.

In a clean bowl, beat the egg whites until soft peaks form. Gradually add the sugar, continuing to beat until stiff peaks form. If desired, add lilac food coloring and gently fold to combine.

Pipe or spoon the lilac meringue onto the lemon filling in each tart. Use a kitchen torch to lightly brown the meringue or place the tarts under a broiler for a few minutes, watching closely to prevent burning.

Garnish the tarts with fresh mint leaves if desired. Serve immediately and enjoy.