How to Make It
Preheat the oven to 300°. Spread the nuts in a pie plate and toast them in the oven until golden brown, about 20 minutes. Transfer the nuts to a plate and let them cool completely. Raise the oven temperature to 350°.
In a food processor, combine the flour with the milk chocolate and process until the mixture resembles coarse meal. Add the sugar, cocoa powder and salt and pulse to mix. Add the beaten whole eggs and the vanilla and process until a dough forms. Transfer the dough to a lightly floured surface and knead in the toasted nuts.
Divide the dough in half. On a lightly floured work surface, roll each piece of dough into a foot-long log. Transfer the logs to a baking sheet lined with parchment paper or foil and flatten them slightly. Lightly brush the logs with the beaten egg white and bake for about 30 minutes, or until firm to the touch and slightly cracked. Remove from the oven and let cool for 1 minute. Turn the oven down to 325°.
Transfer the hot logs to a work surface. Using a serrated bread knife or a sharp chef's knife, slice the logs 1/3 to 1/2 inch thick on the diagonal. Arrange the slices upright on the baking sheet and bake for about 5 minutes, or until dry. Transfer the biscotti to a wire rack and let cool completely.
Line a baking sheet with wax paper. Dip half of a cookie into the tempered chocolate, letting the excess drip back into the bowl, and set the cookie on the wax paper, cut side down. Repeat with the remaining cookies. Let stand in a cool, dry place until set.
Make Ahead
Notes
Tempering Tips Tempering melted chocolate ensures that it will set up firmly with a glossy sheen. Any leftover chocolate can be tempered again later.
1. Chop 1 pound of room temperature chocolate into 1/2–inch pieces.
2. Put 2/3 of the chocolate in a glass bowl. Microwave at medium–high power for 30–second periods, stirring with a rubber spatula in the intervals, until 3/4 of the chocolate is melted, about 1 1/2 minutes.
3. Stir the chocolate until completely melted, then scrape it into a clean, dry, room temperature bowl. Measure the temperature of the chocolate with an instant–read thermometer; it should be between 100 and 115 degrees. Add the remaining chocolate in large handfuls, stirring constantly until the chocolate is at 88 degrees. If the chocolate becomes too cool, melt a few more tablespoons of chopped chocolate and stir them in to raise the temperature.
4. Dip the tip of a knife in the chocolate; the chocolate on the knife should begin to set within 1 minute. Use at once.