Make the Tart Shells: In a food processor, combine all-purpose flour and powdered sugar, pulsing until blended. Add cold unsalted butter and pulse until it resembles coarse crumbs. Incorporate the egg yolk and water, pulsing until the dough just forms. Wrap in plastic wrap and chill for 30 minutes. Preheat oven to 350°F (175°C).

Shape and Bake the Tart Shells: Roll out the chilled dough, press into mini tart pans, prick the base, line with parchment and fill with weights. Bake for 10-12 minutes until golden brown, then cool.

Prepare the Lemon Curd: Whisk lemon juice, zest, sugar, and eggs in a saucepan over medium heat for about 8-10 minutes until thickened. Remove, stir in butter until smooth, and cool slightly before filling tart shells. Refrigerate for at least one hour.

Make the Lilac Meringue: In a heatproof bowl over simmering water, whisk egg whites, sugar, and cream of tartar until warm. Move to an electric mixer and beat for 5-7 minutes until stiff peaks form. Fold in vanilla and food coloring.

Assemble and Garnish: Pipe meringue onto cooled lemon curd tarts and lightly toast with a kitchen torch. Garnish with edible lilac petals or dried lavender.