Preheat the oven to 180°C (160°C fan-forced). Lightly grease 2 baking trays and line with baking paper.
Using an electric stand mixer, combine butter, sugar, zest, brandy, and vanilla together using the paddle attachment, beating until pale and fluffy. Add eggs one at a time, beating until just combined. Stir in flour, cocoa, baking powder, salt, pistachios, cranberries and chocolate until just combined. Cover dough and chill in refrigerator for 30 minutes.
Halve dough and form two logs, approximately 6cm x 25cm. Place on prepared baking trays and bake in preheated oven for 30 minutes or until logs are lightly golden. Remove from oven and allow to cool on baking trays for 15 minutes, reducing oven temperature to 140°C (120°C fan-forced).
Transfer logs to a chopping board and using a sharp knife cut 1cm thick slices. Place biscuits cut side down on baking trays and return to oven for 20 minutes or until dried out completely, turning after 10 minutes. Remove from oven and allow to biscotti to cool on a wire rack.
NOTE: these biscuits make great edible Christmas gifts, as they will last in an airtight container for a month.