In a food processor, pulse together the all-purpose flour and powdered sugar until mixed. Add the cold unsalted butter, blending until crumbly. Incorporate the egg yolk and enough cold water to form a dough. Chill for at least 30 minutes.

Preheat the oven to 350°F (175°C). Roll out the chilled dough, press into mini tart pans, prick the bases, line with parchment and weights, and bake for 10-12 minutes until golden. Allow to cool completely.

In a medium saucepan, whisk together lemon juice, lemon zest, granulated sugar, and eggs. Cook over medium heat, stirring constantly for 8-10 minutes until it thickens. Remove from heat and whisk in unsalted butter until smooth. Pour into cooled tart shells and chill for 1 hour.

Set up a heatproof bowl over simmering water. Combine egg whites, granulated sugar, and cream of tartar. Whisk until warm and sugar dissolves, about 5 minutes. Beat on high until stiff peaks form. Fold in vanilla extract and violet food coloring, then pipe onto chilled lemon tarts.

Use a kitchen torch to toast the tips of the lilac meringue until golden brown. Alternatively, use a broiler briefly but monitor closely. Garnish with edible lilac petals or dried lavender.