Preheat oven to 350° with rack in top third of oven. Lightly grease a cookie sheet or line it with parchment paper.
Thoroughly mix together Chocolate Brownie Mix and oil. Add egg and water, and mix again. Stir in chocolate chips and nuts, if using.
Drop rounded 1 to 1½ tablespoons of dough per cookie onto cookie sheet. Use bottom of glass or measuring cup to flatten each cookie to ½ to ¾ inch thickness.
Bake 11 to 13 minutes for smaller cookies, 13 to 15 minutes for larger cookies, until they are slightly firm to the touch, and a few cracks appear on top. Let cool on pan before removing with spatula.
NOTE: This dough can be frozen and baked later. To do so, form cookie dough into a log 2½ to 3 inches in diameter, wrap tightly in plastic wrap, and freeze. To bake, remove the plastic wrap, and let thaw until dough is soft enough to cut through (about 10 minutes). Cut log into ¼-inch slices and put on prepared cookie sheet. Bake for 1-2 minutes longer than as directed above.
For high altitude baking, increase oven temperature to 375° and bake according to recipe directions.
To prevent the bottoms of these chocolatey cookies from getting too dark, it is important to bake them in the top third of oven, or to use an insulated cookie sheet.