Preheat oven to 350° with rack in middle of oven. Lightly grease a cookie sheet or line it with parchment paper.

In bowl of stand mixer with paddle attachment, mix Chocolate Cake Mix and butter together. In separate bowl, whisk egg and water together, then add it to cake mix and beat until completely mixed.

Scoop one tablespoon dough per cookie onto cookie sheet, roughly patting each into a ball before flattening to ½ to ¾ inch thick. Bake 10 to 12 minutes, until cookies are firm but not hard to the touch. Let cool on pan placed on rack.

NOTE: 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 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.