Frangipane, a mixture of ground nuts, butter, sugar and egg, is typically the base of fruit tarts, but here a hazelnut version encapsulates chocolate chunks for a tart that tastes like a rich chocolate chip cookie. It’s the kind of dessert that you want to come back to over and over again throughout the week to shave off thin snack-size slivers.
Because the hazelnuts make it pretty rich, cut the tart into smaller wedges than usual if serving to guests. The texture of the filling improves dramatically with an overnight rest, so plan accordingly. And when making it, be sure your butter is at room temperature or the filling won’t come together.
Make the pastry dough: In the bowl of a food processor or in a large bowl, combine the flour and salt and pulse or whisk to combine. Add the butter and pulse until it forms pea-sized crumbles in the flour, about 10 pulses; or rub the butter into the flour with your fingers. Mix the water and egg yolks together in a bowl, then add to the flour and butter and pulse just until the crumbles are moistened, about 6 pulses; or use a fork to toss the flour and butter with the water and yolks in the bowl until the dough starts to come together.
Scrape the dough crumbles onto a work surface and knead briefly to combine; the dough might seem a little dry now but resist the urge to add more liquid since it will hydrate fully while resting in the refrigerator. Shape the dough into a 3/4-inch-thick disk and wrap in plastic wrap. Refrigerate at least 1 hour or up to 2 days.
Meanwhile, make the filling: Heat the oven to 325 degrees. Spread the hazelnuts on a baking sheet and bake, stirring halfway through, until deeply bronzed and fragrant, 12 to 14 minutes. If your hazelnuts still have their skins, transfer the warm nuts to a kitchen towel, gather the corners to enclose them and rub the hazelnuts through the towel to loosen and remove the skins as best as you can (but don’t lose sleep over it). Transfer the sheet to a rack and let the hazelnuts cool completely.
Transfer the nuts to a food processor and pulse until finely ground, about 20 one-second pulses. Add both sugars and pulse until evenly combined and the hazelnuts are very finely ground, about 20 more one-second pulses. Add the softened butter, vanilla, salt and egg and process until the mixture is smooth and has the consistency of nut butter, 20 to 30 seconds. Scrape down the side of the bowl and process for about 5 seconds more to make sure the mixture is smooth throughout. Scrape the frangipane filling into a bowl, add the chocolate and flour and stir to combine (the filling will be thick). Cover the filling with plastic wrap and store at room temperature until ready to use.
Remove the dough disk from the refrigerator and transfer to a floured work surface. Using a floured rolling pin, flatten the dough into a round that is 1/8-inch thick, about 14 inches in diameter. Transfer the round to a 9 1/2-inch fluted tart pan and allow it to gently sink into the pan, pressing it lightly against the corner and up the sides. Place your rolling pin over the pan and roll it over the edges to trim the excess pastry. Transfer the pan to the refrigerator to chill for 20 minutes.
While the dough is chilling, line a baking sheet with foil (not parchment paper) and place it in the oven, then increase the oven temperature to 375 degrees.
Crumple a sheet of parchment paper into a ball, unfurl, then use it to line the chilled pastry crust. Fill the crust with pie weights or dried rice or beans, making sure they reach the corners of the pan. Place the pan on the baking sheet in the oven and bake until the edge of the crust is lightly browned, about 25 minutes. Remove the sheet from the oven and then remove the parchment paper and weights from the crust. Reduce the oven temperature to 325 degrees.
Scrape the filling into the crust, let sit for a couple seconds, then use an offset spatula or spoon to gently coax the filling into an even layer; it does not need to be completely smooth on top. Sprinkle with a big pinch of flaky sea salt. Return the tart on the baking sheet to the oven and bake, rotating the sheet halfway through, until the filling is puffed and golden brown all over like a giant chocolate chip cookie and a toothpick inserted in the center comes out clean, 35 to 40 minutes.
Remove the sheet from the oven and let the tart cool completely. Let the tart rest for at least 8 hours or overnight because, like a chocolate chip cookie, its texture improves after a long period of rest (don’t ask me why, just do it!).
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