Preparation

Make the crust topping:

• Lightly grease a 9×13-inch baking pan. Line the bottom and sides with a sheet of parchment, leaving a few inches of paper hanging over the edges of the two long sides to help remove the finished bars.
• Roughly chop 1/2 cup of the hazelnuts; halved or quartered is about right. Set aside. Put the remaining 1/2 cup hazelnuts in a food processor and process until finely ground; put them in a large bowl. Add the flour, confectioners’ sugar, and baking powder and stir with a fork to blend. With a pastry blender or two knives, cut in the butter until the largest lumps are the size of fat peas. Lightly beat the egg and yolk and add all but 2-tablespoons of the egg to the butter-flour mixture. Add the vanilla and gently toss with your fingertips until the mixture clumps together. It should be uniformly blended, moist, and crumbly. If the dough looks dry and floury, mix in a little more beaten egg. Or you can mix the dough in a large (11-cup capacity) food processor. Pulse in short bursts to avoid overworking the dough.
• Heat the oven to 350°F. Divide the dough into two portions, one slightly larger than the other. Using floured fingertips, pat the larger portion into the bottom of the prepared pan, pressing firmly, and refrigerate for 15 to 20 minutes. Cover the remaining portion with plastic wrap and refrigerate as well.
• Remove the pan from the fridge and prick the crust all over with a fork. Bake until the edges are golden and the pastry is dry and light brown, 20 to 25 minutes. Let cool completely on a wire rack before filling (otherwise, the bars will get soggy).

Make the filling:

• In a large bowl, combine the berries, sugar, cornstarch, lemon juice, cinnamon, and salt. Toss gently to thoroughly coat the berries and then scatter the filling onto the cooled crust, scraping the bowl well. Spread the berries in an even layer.

  Crumble the reserved dough over the berries; it won’t cover everything. Bake for 25 minutes and then sprinkle on the reserved chopped hazelnuts. Continue baking until the topping is puffed and golden and the filling is bubbling not only near the edges but also close to the center of the pan, another 50 to 60 minutes. Put the pan on a wire rack and let cool completely. Use the parchment to lift the whole batch out and then cut into bars. A long serrated knife works well; jiggle it a little through the top layer and then gently push down to cut through the bottom crust.