In a medium bowl, whisk together the egg yolks and sugar until the mixture is light yellow. Whisk in the cornstarch and set aside.
In a medium saucepan over medium heat, whisk together the milk, dulce de leche and vanilla seeds, and bring almost to a boil.
In a thin, steady stream, slowly add all the hot milk mixture to the egg mixture, whisking constantly. Pour it all back into the pan, return to the heat and cook, stirring constantly, until very thick, about 10 minutes. Remove from the heat and stir in the butter until melted and mixed in. Strain through a fine-mesh sieve into a bowl and place a piece of plastic wrap directly on the surface. Let cool completely, then store in the fridge.
Beat the pastry cream in a stand mixer fitted with the paddle attachment until smooth before using.