Preheat your oven to 180°C (350°F).
In a bowl, mix the softened butter and powdered sugar until creamy.
Add the egg, almond flour, and salt. Mix well.
Gradually incorporate the all-purpose flour until a dough forms.
Press the dough into a tart pan and bake for 15 minutes or until golden.
In a saucepan, heat the milk and vanilla extract over medium heat until warm.
In a separate bowl, whisk the egg yolks, granulated sugar, and cornstarch.
Slowly pour the warm milk into the egg mixture while whisking continuously.
Return the mixture to the saucepan and cook over low heat, stirring constantly, until thickened.
Pour the custard into the baked tart shell.
Cut the rhubarb into small pieces and simmer with water, sugar, lemon juice, cinnamon, and star anise until tender.
Drain the rhubarb and arrange it over the custard.
Chill the tart for at least 1 hour before serving.