Portuguese Custard Tarts - Pasteis de Nata

[Lou-lou]

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Ingredients
1 cup milk
3 tablespoons cornstarch
1/2 vanilla bean
1 cup white sugar
6 egg yolks
1 (17.5 ounce) package frozen puff pastry, thawed
Directions
Preheat oven to 375 degrees F (190 degrees C.) Lightly grease 12 muffin cups and line bottom and sides with puff pastry.
In a saucepan, combine milk, cornstarch, sugar and vanilla. Cook, stirring constantly, until mixture thickens. Place egg yolks in a medium bowl. Slowly whisk 1/2 cup of hot milk mixture into egg yolks. Gradually add egg yolk mixture back to remaining milk mixture, whisking constantly. Cook, stirring constantly, for 5 minutes, or until thickened. Remove vanilla bean.
Fill pastry-lined muffin cups with mixture and bake in preheated oven for 20 minutes, or until crust is golden brown and filling is lightly browned on top

[Colleenconti]

At what temperature do you cook the Custard mixture? Medium high? Simmer? Boil?