Bake 6-8 medium-sized sweetpotatoes at 400°F for 1 hour. Sweetpotatoes should be soft and fragrant. Set aside to cool.
Peel and mash sweetpotatoes in a medium-sized bowl. Measure out two cups of sweetpotato, mash it up and set aside. (Mash should be room temperature).
Toast the pecans, if not already toasted. Lower oven temperature to 375°F and bake nuts on a clean sheet pan for 8-10 minutes until dark brown. Take care not to burn them. They should be fragrant but smell burned.
Lower oven temperature again to 350°F. Grease a 9 X 13″ pan with softened butter.
Using a whisk, combine all dry ingredients except sugar in a large bowl, set aside.
In another large bowl, combine sweetpotato mash, sugar and buttermilk. Add eggs one at a time and whisk until combined. Slowly stream in oil, and whisk until combined. Add wet mixture/sweetpotato into dry ingredients, folding in together.
Once ingredients are almost combined, add pecans and chocolate. Fold gently until incorporated and being careful not to over mix.
Pour batter into greased cake pan and bake for 45-50 minutes, or until cake is set. It should be dark golden brown on the top and the center of the cake should spring slightly when pressed.
Cool completely for 20-30 minutes before serving.