Line a jelly roll pan with parchment and keep at hand. Remove the bottoms from six 4 1/2 inch fluted tartlet pans (or use pans with permanent bottoms and just plan to pop the tartlets out once they're filled, baked, and cooled); spray the pans with vegetable oil spray or brush with melted butter. Cut the dough into six even pieces. Working with one piece at a time, shape the dough into a rough circle, then tamp it down with a rolling pin. Flour the work surface and the top of the dough and roll it into a circle 1/8 to 1/4 inch thick. As you roll, lift the dough with the help of a dough scraper to keep it from sticking. If the dough breaks (as it sometimes does even when the pros are rolling it), press it back together and keep going- it will be fine once it's baked. Fit the dough into a tartlet ring, pressing it into the fluted edges and cutting the top level with the edges of the pan. Again, patch as you go. Use a pastry brush to dust off any excess flour and place the lined tartlet ring on the prepared baking pan. When all of the shells are rolled out, chill them for at least 20 minutes. (Please see note in my Chocolate Tart Dough recipe regarding this step.) Center a rack in the oven and preheat the oven to 350°F. Prick the bottoms of the crust all over with the tines of a fork and bake for 12-15 minutes, rotating the pan halfway through the baking time, until the crusts are dry, blistery, and firm. Transfer the baking pan to a rack so that the crusts can cool while you make the filling. Reduce the oven temp to 300°F. Julia tells you here to melt the butter and chocolate over a double-boiler on the stove. I use the microwave at half power, checking every 30 seconds after the first minute, and stirring each time. Allow to cool until it is slightly warmer than room temperature. Put the yolks and vanilla extract in the bowl of a mixer fitted with the whisk attachment. Start beating the yolks at medium speed, and then, when they are broken up, reduce the speed to low and gradually add the sugar. Increase the speed to medium-high and beat the yolks and sugar until the yolks thicken and form a slowly dissolving ribbon when the beater is lifted. Spoon about one third of the yolks onto the cooled chocolate mixture and fold them in with a rubber spatula. Don't worry about being too thorough. Pour the chocolate into the beaten yolks and gently fold the two mixtures together until they are almost completely blended. Add the cubed chocolates and biscotti, folding to incorporate the chunky pieces. Using an ice cream scoop or 1/4 cup measure, divide the filling evenly among the cooled shells. Smooth the filling with a small offset spatula, working it into the nooks and crannies as you circle the tops of the tarts. Bake the tarts for 10-12 minutes, until the tops look dry and the filling is just set. Remove to a rack to cool for about 20 minutes before serving. Best the day these are made, these are still terrific after they've been refrigerated- they lose their textural finesse, but the taste is still very much there. For longer keeping, wrap the tartlets airtight, and freeze them for up to a month. Thaw, still wrapped, at room temperature. Cook's Notes: Instead of individual tartlets, I made a 10-inch tart, and adjusted the baking time a bit longer than what is noted in the recipe. Additionally, when the tart came out of the oven, I wasn't pleased with the way the tops looked- too bumpy. So I made a quick ganache (2 ounces bittersweet chocolate+ 1/3 cup cream, nuked for about a minute, and stirred until smooth), and spread it over the top. Yeah, this was decadent. It was incredible the day it was served. The remainder was frozen and pulled out at the last minute for an impromptu guest dessert. I let it thaw at room temp for about 20 minutes before serving. I think I liked it better that way. Either way, it's an extravagant gut-bomb that is certain to please.