Preheat oven to 220°.
Line a baking tray with baking paper.
In a medium bowl cream butter with sugar, add flour, cardamom and blend until combined. Add eggs, and mix all together. Fold in orange zest and walnuts.
On the prepared baking sheet, shape the dough into 2 (2x12 inches) logs. Bake for about 25 minutes or until lightly browned.
Cool on the baking sheet for 10 minutes and reduce oven temperature to 200°.
Slice the partially cooled logs using a knife on a slight angle into 1/2 inch thick slices.
Place slices with cut side down on a baking sheet and bake for about 15 minutes or until biscotti are dry on top and slightly toasted at the bottom. Turn over and bake for 15 more minutes.