Preheat oven to 350°F. Oil and lightly flour a 9x5 inch loaf pan.
In a small bowl, combine the flours, starch, guar gum, cocoa powder, baking soda, baking powder and salt.
In a separate bowl, cream together the butter and sugar until very light and fluffy. Add the eggs, one at a time, beating well after each addition, then add the vanilla extract.
Add the dry ingredients to the butter mixture, alternating with sour cream. Mix until combined. Add chocolate chips, if desired.
Pour into prepared pan. Bake at 350°F for 40 - 50 minutes until a toothpick inserted in the middle of the cake comes out clean and the edges begin to pull away from the sides. Let cool.