Preheat oven to 350.
In a large bowl, cream together butter and white and brown sugars, by hand or with a mixer. Gradually add eggs, one at a time, then add molasses and 2 Tb candied ginger and mix well. Reserving other 2 Tb candied ginger for topping
Mix together flour and all spices. Gradually add flour mixture to butter mixture, mixing well, until dough is slightly stiff but still soft.
Turn out dough onto a lightly floured surface and form into two small rectangular loaves. Place on a large baking sheet lined with parchment paper or a silicon baking mat and bake for 20 minutes. Remove from oven and let cool on sheet for 5 minutes.
Reduce oven to 300 degrees. Remove biscotti loaves from pan and cut into ½-3/4" slices on the diagonal with a sharp knife and place slices on baking sheet. Bake for additional 15-20 minutes. Remove from oven and transfer biscotti to wire rack and let cool completely.
Once biscotti has completely cooled, dip one side of each slice into melted chocolate and sprinkle with the reserved chopped candied ginger. Place back on wire rack to dry. Store leftovers in covered container for 4-6 weeks.