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Preheat oven to 375°F. Line a baking sheet (12- X 17-inches) with parchment or non-stick liner.
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Whisk together flour, baking powder, cinnamon, cardamom and salt in a medium bowl.
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Cream butter, shortening, and sugar in a large bowl. Add eggs and vanilla, one at a time, mixing well after each addition.
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Add flour mixture and mix until just combined. Add walnuts and currants and mix until distributed throughout the dough.
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Divide dough into 3 equal pieces. Shape each piece into a log 11 1/2 inches long.
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Transfer logs to baking sheet with 3 inches between them. Pat the logs into rectangles, 12 inches long, 2 1/2-inches wide and 7/8-inch thick.
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Bake logs for 20 to 25 minutes, until the edges are just beginning to brown.
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Remove logs from the oven and let rest for 20 minutes.
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Reduce oven temperature to 300°F. Line another baking sheet (12- X 17-inches) with parchment or non-stick liner.
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Gently transfer a log to a cutting surface. Use a serrated knife in a slow, gentle sawing motion to cut the log into about 1/2-inch slices on the diagonal (my slices were slighty wider than 1/2-inch).
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Carefully tranfer the slices, cut side up, to the baking sheet. Place slices 1/4-inch apart. Repeat with remaining 2 logs.
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Bake for 20 minutes. Remove from the oven and quickly turn slices over.
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Return biscotti to the oven and bake for 20 minutes, or until they are very dry and lightly brown.
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Remove biscotti from the oven and cool completely. Store in a sealed container.