Preheat the oven to 350°F (175°C). In a mixing bowl, combine flour, butter, and powdered sugar until crumbly. Add egg yolk and mix until a dough forms. Press into tart pan and bake for 15 minutes.

In a saucepan, combine lemon juice, sugar, egg yolks, lilac syrup, and cornstarch. Cook over medium heat until thickened, stirring constantly. Pour into baked crust.

In a clean bowl, whisk egg whites until soft peaks form. Gradually add sugar and continue whisking until stiff peaks form. Spread over the filling.

Bake the tart in the oven for an additional 10 minutes, or until the meringue is golden brown. Allow to cool before serving.