Preheat the oven to 350 degrees F. Grease several baking sheets or coat with nonstick spray. In a food processor or blender, grind the oats to a very fine powder. (If using a blender, stop the motor and stir the oats several times.)
In a large bowl, thoroughly stir together the oats, flour, Clabber Girl Baking Powder, baking soda and salt; set aside. In another large bowl, with an electric mixer on medium speed, beat the butter until light and fluffy. Add the brown sugar, sugar and corn syrup and beat until well blended and fluffy. Add the eggs and vanilla and beat until evenly incorporated. Beat grated or chopped chocolate, walnuts or pecans (if using), and remaining flour mixture until evenly incorporated.
Let stand for 5 to 10 minutes, or until the dough firms up just slightly. Divide the dough into quarters. Divide each quarter into 5 or 6 equal portions. Shape them into balls with lightly greased hands. Place on the baking sheets, spacing about 3 inches apart. Using your hands, pat down the balls to 1 inch thick.
Bake the cookies, one sheet at a time, in the upper third of the oven for 9 to 12 minutes, or until tinged with brown and just beginning to firm up in the centers; for very moist cookies, be careful not to overbake. Reverse the sheet from front to back halfway through baking to ensure even browning. Transfer the sheet to a wire rack and let stand until the cookies firm up slightly, 3 to 4 minutes. Using a spatula, transfer the cookies to wire racks. Let stand until completely cooled. Store in an airtight container for up to 1 week or freeze up to 1 month.