Preheat the oven to 350°F (175°C). In a mixing bowl, combine flour and powdered sugar. Cut in butter until mixture resembles coarse crumbs. Stir in egg yolk until dough forms. Press into tart pan and bake for 15 minutes.

In a bowl, whisk together lemon juice, sugar, egg yolks, melted butter, and lemon zest. Pour into the baked crust and bake for an additional 10 minutes.

In a clean bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar, continuing to beat until stiff peaks form. Stir in lilac extract.

Spread meringue over the lemon filling, making sure to seal the edges. Bake for 10 minutes or until meringue is lightly browned.