Combine all-purpose flour and powdered sugar in a food processor, pulsing until well mixed. Add cold unsalted butter and process until crumbly. Mix in egg yolk and enough water to form a dough. Chill for 30 minutes, then roll out, line mini tart pans, prick the bases, and bake at 350°F for 10-12 minutes until golden. Cool completely.

In a saucepan over medium heat, whisk together lemon juice, zest, sugar, and eggs until thick, about 8-10 minutes. Stir constantly to prevent curdling. Mix in butter until smooth. Pour into cooled tart shells and chill for at least one hour.

Whisk egg whites, sugar, and cream of tartar in a heatproof bowl over simmering water until warm and sugar dissolves. Beat with an electric mixer until stiff peaks form and glossy, about 5-7 minutes. Fold in vanilla and optional food coloring.

Carefully toast the tips of the lilac meringue using a kitchen torch. Garnish with edible lilac petals or dried lavender just before serving.