Step 1In 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 2Remove 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.
Step 1Heat the oven to 350 degrees. Put the butter in the freezer for 15 minutes. Stir together the flour, 20 grams sugar, baking powder and salt in a mixing bowl.
Step 2Using a box grater, grate the chilled butter into the flour mixture, and then rub it into the flour with your fingers until you cannot see the butter anymore. Add the milk to the flour mixture a little at a time, stirring with a fork, until the dough holds together.
Step 3Place the dough on a dry work surface dusted with flour. Roll it out to a thickness of about 1/8 inch. Place the dough into the refrigerator for 10 minutes and allow to rest. Line four small (4 3/4-inch) tartlet pans with the rolled-out dough and return it to the refrigerator for 10 minutes (if you are short on dough, roll the scraps out to give you enough for the last tartlet).
Step 4Cut wax paper circles a little bit larger than the tarts. Line the tart shells with the paper and fill them with dried beans or pie weights. Bake for 25 minutes to set the crust. Empty the beans and remove the wax paper, and return to the oven for 5 more minutes, until fully baked. Cool completely and reserve until needed.
Step 5Divide the chilled pastry cream between the tart shells and use a spatula to flatten the top. Sprinkle the remaining 6 tablespoons of sugar evenly over the top of the custards, and brown the tops using a small blowtorch.