Preheat oven to 325 degrees F. In work bowl of an electric mixer combine flour, cocoa powder, sugar, baking soda, salt and espresso powder using paddle on low speed. Add nuts and semisweet chocolate chips. Lightly whisk eggs and vanilla and add to flour mixture at low speed. Mix just until dough comes together. Turn out dough and knead lightly to work any unincorporated dry ingredients into dough. Form into 4 logs, each 2 inches in diameter. Place on two parchment-lined cookie sheets and bake for 30 to 35 minutes. Logs should be firm, with cracks in top and dough should be completely dry to touch. Reduce oven temperature to 300 degrees F. Let logs cool slightly then cut them on a slight diagonal into 3/4-inch thick slices. Lay them flat on cookie sheets and bake for 30 minutes more. Meanwhile, melt white chocolate for dipping in top of a double boiler or in microwave, stirring until smooth. Cool biscotti completely on racks before dipping in white chocolate. Place finished biscotti on parchment-lined cookie sheets in a cool place until chocolate solidifies. Store in an airtight container.