Whisk together the milk and the egg yolks thoroughly in a bowl.
To a small sauce pan add the sugar, corn starch, and salt.
Pour half of the egg yolk/milk mixture slowly and mix until the corn starch has dissolved (Don’t turn the heat on yet).
Once the corn starch has dissolved, turn the heat on to medium high as you pour in the remaining mixture, stirring constantly until it thickens and boils.
Stir one minute and remove from heat the stir in the vanilla extract.
Allow to cool for about 10 minutes, covered with kitchen towel.
Quickly warm the buns in the oven at 200F for 5 minutes.
If using closed buns, using a knife or kitchen scissors, gently poke a hole in the side of the donut to create a pocket inside.
Scoop into a zip lock bag for piping into the brioche buns.
Scoop out chocolate ganache and glaze the top of the buns. Enjoy fresh, or later on.