Step 1 In a saucepan, bring the milk, half of the sugar, coffee beans and vanilla bean to a simmer. Remove from the heat and cover with a piece of plastic wrap, and set aside for 20 minutes at room temperature.
Step 2 Remove the plastic wrap and bring the milk back to a simmer. Meanwhile, beat the remaining sugar and egg yolks in a mixing bowl with a whisk until light and lemon-colored. Pour a little of the simmering milk into the eggs and sugar, mixing constantly. Beat in the cornstarch, a little at a time. Continue to slowly whisk the hot milk mixture into the yolks and sugar until fully incorporated. Pour the mixture back into the saucepan and cook, stirring constantly, over medium heat until the custard thickens, 2 to 4 minutes.
Step 3Strain the thickened mixture into a mixing bowl and whisk for 2 minutes to cool to about 130 degrees. Whisk in the butter, a piece at a time, until well incorporated. Cover with plastic wrap, pressing it against the surface of the pastry cream to prevent it forming a skin and refrigerate until needed, preferably overnight. This makes about 2 cups pastry cream, more than is needed for 4 tarts; the pastry cream will keep, covered and refrigerated, up to 5 days.