Heat oven to 325°F.
In a large bowl, whisk together the flour, cocoa powder, baking powder and salt. Set aside.
In another large bowl, combine the butter and sugar. Beat on medium-high speed until smooth and creamy, about 3 minutes. Add eggs and vanilla and beat until fully blended. On low speed, add nuts and chocolate chips. Gradually beat in flour mixture until a stiff dough forms.
Shape dough into 2 logs, each 10 inches long. Place on ungreased baking sheet; flatten each log to 3 inches wide. Bake at 325° for 35 minutes until set and tops are slightly cracked. Transfer logs to wire racks; cool 15 minutes.
With serrated knife, cut logs into 1/2-inch-thick slices. Place on baking sheet, cut sides down. Bake at 325°F for 20 minutes. Cool biscotti completely on wire racks.
In a microwave-safe bowl, combine the white chocolate and shortening. Microwave on high for 1 minute; stir. Microwave at 30-second intervals, stirring until smooth. Allow to cool slightly. Dip one end of each biscotti into melted chocolate and sprinkle with read and green nonpareils. Place on waxed-paper-lined baking sheets until chocolate is set.