How to Make It
In the bowl of a stand mixer, whisk the water with the yeast until dissolved. Add the ½ cup of bread flour and, using a wooden spoon or your hand, mix until the dough comes together. Cover the bowl with plastic and let rest at room temperature until doubled in bulk, about 4 hours.
Attach the paddle to the stand mixer. Add the bread flour, water, sugar, and egg yolks to the bowl of the stand mixer with the biga in it. Beat on low speed until just combined, about 2 minutes. Mix on medium-high speed until the dough balls up around the paddle and pulls away from the side of the bowl, about 8 minutes. Switch to the dough hook and, with the mixer on low speed, gradually add the butter one tablespoon at a time until incorporated and the dough is smooth and shiny, about 5 minutes. Cover the bowl with plastic and let rest at room temperature for 12 hours or overnight.
Add the bread flour and sugar to the bowl of the stand mixer. Attach the dough hook and beat on low speed until the dough is smooth and elastic, about 5 minutes. With the mixer on low, gradually add the egg yolks and then the vanilla paste until the dough is smooth and elastic again, about 5 minutes more. Add the salt and beat for 2 minutes. With the mixer on low, gradually add the butter one tablespoon at a time until incorporated and the dough is shiny and very supple, about 5 minutes more. Beat in the hazelnuts and chocolate until evenly distributed. Cover the bowl with plastic and let rest at room temperature for 1 hour.
Put two 6-inch panettone molds on a large baking sheet. Scrape the dough out onto a floured work surface and cut it in half. (The dough should be very soft and supple.) Using a bench scraper and floured hands, gently shape the dough into 2 rounds, carefully folding the dough under itself. Carefully transfer the rounds seam side down to the panettone molds. Cover the molds with plastic and let rise in a warm place until the rounds are 1 to 2 inches from the top of the mold, 4 to 6 hours.
Preheat the oven to 350° and position the rack in the center. Lightly brush the tops of the panettone with the egg wash, then sprinkle with pearl sugar. Bake for about 1 hour, until risen and the top is well-browned. Let cool completely, about 1 hour and 30 minutes. Remove the panettone from the molds, cut into wedges, and serve.
To roast and skin hazelnuts, preheat the oven to 375°. Spread the hazelnuts on a baking sheet and toast for about 7 minutes, until golden. Let cool slightly, then transfer them to a kitchen towel and rub off most of the skins.
Panettone molds are available at baking supply stores or at Sur la Table.