In a large mixing bowl, beat the brown sugar, 1 cup of the granulated sugar and the eggs with an electric mixer on medium speed until creamy. Add the oil and sour cream. Beat on low speed until mixed. Add the flour, cocoa, baking powder, baking soda and salt. Beat on low speed until moistened. Then, beat on medium speed until smooth. Cover and refrigerate for at least 3 hours or until the mixture holds its shape. Heat the oven to 325 degrees. Spray large cookie sheets with non-stick cooking spray. Place the remaining 1/2 cup granulated sugar and the powdered sugar in separate bowls. Form 1-inch balls of the cookie dough and drop into the granulated sugar. Roll to coat completely. Then, drop the cookie ball into the powdered sugar and roll to completely cover. Place 2 inches apart on the cookie sheets. For best results, bake one sheet at a time on the center rack. Bake for 10 to 12 minutes or until the edges are set and the top is slightly springy. Remove to cooling racks. Cool for 5 minutes before serving.