shortening together until well blended and soft. Then beat in the brown and granulated sugars in turn, beating well after each addition, until fluffy. Add the eggs one at a time, beating well after each; then beat in the vanilla. Sift together the flour, baking soda, baking powder, and salt. Stir the flour mixture into the creamed mixture, blending well. Add the chocolate bits, reserving 1/2 cup, and mix well. Wrap the dough in plastic wrap the chill it thoroughly, preferable for 3 hours or longer. (It keeps well for at least three days.) When dough is ready to bake, preheat oven to 325, with the oven rack in the center position. Line a 12 inch pizza pan with two sheets of regular-weight aluminum foil, crossing the sheets at right angles to cover the baking surface completely. Crimp the foil securely over the rim and cut away the excess. Grease the foil lightly, then trace a 10-inch circle in the coating, keeping it a uniform 1-inch away from the rim.Set the pizza pan on a large baking sheet. Measure 3 level cups of the dough into the center of the pattern on the baking pan (use remainder for smaller cookies). Spread dough evenly over the marked area. Don't smooth top. Sprinkle the extra chocolate chips over the top and press them in about halfway. Bake 25 minutes, then cover loosely with foil and bake an additional 15 to 20 minutes, until cookie has puffed and fallen back and feels barely firm when touched. When cookie is done, remove it, in it's pan, from the baking sheet and cool it completely on a wire rack. Makes a 12 inch cookie with leftovers for making smaller cookies