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1
Preheat oven to 300u00b0F Line a large baking sheet with parchment paper (waxed paper will not work, it will smoke.).
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2
In a food processor fitted with the metal blade, combine the chocolate and sugar and grind until the chocolate is very fine. Set aside.
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3
In a bowl, sift together the flour, cocoa powder, baking soda and salt. Set aside.
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4
In a mixing bowl combine the eggs and the vanilla. Beat at medium speed to blend.
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5
Reduce the speed to low, add the chocolate and flour mixtures and mix until a stiff dough forms, adding the walnuts when about half mixed.
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6
Transfer the dough to a floured surface and gather it together. Divide in half. Form each half into a log 12 inches long. (Give the logs plenty of space on both sides as the width of the logs doubles during baking.).
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7
Carefully transfer the logs to the prepared baking sheet, spacing them evenly.
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8
Pat to even up the shapes. Bake until almost firm to the touch, about 50 minutes. (I have found that 40 minutes was sufficient).
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9
Let cool for 10 minutes. (Do not let it cool for longer as the logs will become brittle and more difficult to slice without breaking.).
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10
Leave the oven set at 300u00b0F.
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11
Using a spatula, transfer the logs to a work surface. Using a serrated knife, cut on the diagonal into slices 1/2 to 3/4 inch thick. Return the slices cut side down to the baking sheet.
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12
Bake for 25 minutes. (I baked mine 15 minutes and the biscotti were very crispy).
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13
Turn the slices over and bake until crispy and dry, about 25 minutes longer. (Again, I found 15 minutes to be good).
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14
Transfer the cookies to wire racks to cool.
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15
Store in an airtight container at room temperature for up to 1 month.
