Pulse together flour and powdered sugar in a food processor until well combined. Add cold, cubed butter and process until resembling fine crumbs. Mix in egg yolk and a splash of cold water until it comes together. Chill for 30 minutes, then roll out and fit into mini tart pans. Bake at 350°F for 10-12 minutes until golden brown. Allow to cool completely.

In a saucepan, whisk together lemon juice, lemon zest, sugar, and eggs. Over medium heat, stir constantly until thickened (8-10 minutes). Stir in butter until melted and smooth. Pour into cooled tart shells and refrigerate for at least 1 hour to set.

Set up a double boiler with a heatproof bowl over simmering water. Combine egg whites, sugar, and cream of tartar. Whisk until warm. Transfer to a stand mixer and beat on high until stiff peaks form. Add vanilla extract and violet food coloring, mixing gently. Pipe onto lemon-filled tarts.

Gently toast the tips of the meringue with a kitchen torch until golden. Rotate the tarts as you toast for even coloring. Garnish with edible lilacs or dried lavender if desired. Serve immediately or keep chilled until serving.