How to Make It
To prepare the crust, combine the flour, confectioners’ sugar, cocoa powders, espresso powder, and salt in the bowl of a food processor; pulse to combine well. Sprinkle the butter pieces over flour mixture; pulse until you no longer see any butter pieces and the mixture looks like moist brown sugar. Add in the yolks and pulse until a dough just starts to form.
Dump the mixture onto a clean work surface and using your hands, press it together; divide dough in half. Form each half into a disc, and wrap tightly with plastic wrap. Refrigerate for at least 30-60 minutes. (Once well-chilled, you can transfer crust to a freezer bag and freeze for up to three months for future use; thaw in the refrigerator overnight before using.)
Preheat oven to 350°. Roll out your dough to about a 13-inch circle between two pieces of parchment paper, occasionally peeling back the parchment on both sides so that it doesn’t get stuck. If you don’t have parchment, you can roll on a counter dusted with cocoa powder (so that the crust keeps its dark exterior). Transfer the dough to a 9-inch round tart pan with removable bottom. Trim the edges of the crust even with pan, and carefully line the tart shell with either foil or parchment paper. If using parchment, it can help to crumple it up first to make it more flexible and allow it to ease into the corners. Put the tart shell back into the fridge and chill for at least 20 minutes or up to a day.
Fill the lined shell with pie weights, dried beans, or rice, and bake for about 25 minutes to set the shell. Remove the foil or parchment with the weights and continue to bake for about 10 minutes more until completely cooked. The crust should be matte with no shiny parts, and fairly firm to the touch. Cool completely on a rack before removing the tart pan.
To prepare the filling, whisk the cornstarch and flour in a medium bowl until thoroughly combined. In a separate bowl, combine ½ cup of the milk with the beaten egg yolks and whisk to combine. Add the milk mixture to the cornstarch mixture, whisking until well combined. Set aside.
Create an ice bath by placing a medium glass or stainless-steel bowl into a larger bowl filled with ice water and set aside. Have a mesh strainer or sieve ready.
Combine the remaining 2 ½ cups of the milk, sugar, vanilla paste, and salt in a medium saucepan. Over medium heat, bring the mixture to a simmer, stirring frequently, until the sugar is dissolved completely. Remove from the heat. Slowly add about a cup of the hot milk mixture into the egg mixture, whisking rapidly to prevent curds from forming as you temper the eggs. Pour the tempered egg mixture back into the pan with the remaining hot milk, whisking rapidly as it goes in, and return the pan to medium heat.
Bring the custard to a boil over medium heat, whisking constantly, allowing it to thicken. Once the mixture has thickened, pour it through the strainer into the bowl in the ice bath, pressing the mixture through the strainer to remove any lumps. Let cool for just a few minutes, stirring frequently, until it is no longer steaming hot. Using a rubber spatula, stir in the black tahini paste.
Give the surface of the pastry cream either a very light spray of flavorless cooking spray or rub a small piece of unsalted butter over the surface, and then place a piece of plastic wrap directly on the surface so that a skin does not form. Transfer the covered pastry cream to the fridge to chill completely. This can be made up to five days in advance.
Before using, allow the pastry cream sit at room temperature for about 15 minutes, then whisk vigorously to loosen.
To assemble the tart, gently spread the jam on the bottom of the crust. Top with the tahini pastry cream. Arrange the berries in a single decorative layer on top of the cream. Keep covered and chilled until about 30 minutes before you want to serve. Garnish with whipped cream, dark chocolate shavings or pearls, or a light dusting of black cocoa.
I know a lot of recipes recommend rolling the crust onto your rolling pin to transfer, but I often have it slide off, and this crust is more shortbread cookie-like and can crack, so here is my trick. I slide the removable bottom of my tart shell underneath the rolled-out dough, then fold it over gently about four or five times until it is a neat little package on top of the metal. Then I can just drop it into the tart ring, and gently unfold it and ease it into all of the nooks and crannies of the fluted tart pan. If you get a crack, mend it with excess crust, it is very forgiving.
If you prefer a softer, fluffier textured filling, whip one cup of heavy cream to soft peaks and fold it into your pastry cream after allowing it to sit at room temperature at step 10. It will lighten the color, but the texture and flavor are still delicious.