Add the cubed butter, sugar, salt and vanilla to a stand mixer fitted with a paddle attachment. Mix for about 2 minutes or until it's homogeneous.
Add the egg yolks and mixed until combined.
Add the flour and mix until shaggy, about 45 seconds.
DIVIDE into 2-3 equal pieces and press each into a flat disk. Roll out to a thickness of 3mm, 1/8 inch, in between two sheets of lightly floured parchment paper. Chill for at least an hour.
Peel off the layers of paper and cut your tart bottoms out. Cut a strip for the side. Place the strip for the side in your tart ring. Press the side into the ring and trim the excess.Trim the top so it's flush to the ring.
Freeze tarts for at least half an hour. Bake at 320F for about 12-15 minutes or until just golden. If you're using a traditional tart mold you can just cut the rounds out and press into the form.
For the Cream
In a medium saucepan, heat the milk and vanilla bean to a boil over medium heat. Immediately turn off the heat and set aside to infuse for 15 minutes.
In a bowl, whisk the egg yolks and sugar until light and fluffy.
Add the cornstarch and whisk vigorously until no lumps remain. Whisk in 1/4 cup of the hot milk mixture until incorporated.
Whisk in the remaining hot milk mixture, reserving the saucepan. Pour the mixture through a strainer back into the saucepan. Cook over medium-high heat, whisking constantly, until thickened and slowly boiling. Remove from the heat and stir in the butter.
Let cool slightly. Cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming.
Chill at least 2 hours or until ready to serve. The custard can be made up to 24 hours in advance. Refrigerate until 1 hour before using. I whipped some cream up and folded it into my chilled custard to lighten the filling up, this is now technically called Ambassador Cream.