In a large bowl, cream together butter, sugar, orange zest and vanilla with an electric mixer until light and fluffy. Mix in eggs and egg white one at a time, beating well after each addition. Sift together the flour, baking powder and salt; gradually blend into the creamed mixture using a wooden spoon. Stir in the white chocolate, dried cranberries and pistachios. Cover and chill for 30 minutes, or until dough is no longer sticky.
Preheat oven to 170 degrees C. Line a baking tray with baking paper.
Turn dough out on a lightly floured surface and divide into halves. Form each half into a flattish log about 30cm long by 8cm wide. Arrange logs at least 8cm apart on baking tray.
Bake for 30 minutes in preheated oven, or until pale gold. Allow logs to cool on the baking tray until cool enough to handle. On a cutting board, cut logs crosswise on a diagonal into 3cm thick slices. Arrange on baking tray.
Bake for an additional 15 minutes or until golden. Transfer biscotti to wire racks and cool completely. Store in an airtight container at room temperature.