Meanwhile, make the filling. Put the gelatin in a small bowl; add the boiling water and stir until the gelatin is dissolved. In a medium saucepan, whisk together 1/3 cup
of the sugar and the flour
. In a medium bowl, lightly beat the milk and egg yolks together. Add the egg-and-milk mixture to the saucepan and whisk so the flour and sugar dissolve. Cook over medium heat, stirring constantly, until the mixture comes to a boil and has thickened. Stir in the vanilla and dissolved gelatin. Set aside to cool slightly.