Using an electric mixer, beat butter on medium speed until light and fluffy, about 1 minute. Add egg and water and beat until well blended, about 1 minute. Add cookie mix and beat until dough comes together, 2 to 3 minutes.
For Cutout Cookies: Shape dough into 2 disks. Wrap tightly with plastic wrap and refrigerate until firm, at least 3 hours or up to overnight. Line 2 baking sheets with parchment paper. On a floured work surface, roll out dough 1/4 inch (6 mm) thick, dusting with more flour as needed. Using cookie cutters about 2 inches (5 cm) in diameter, cut out cookies and transfer to baking sheets. Reroll scraps and cut out more cookies.
For Sliced Cookies: Shape dough into 2 logs, each about 1 1/2 inches (4 cm) in diameter and 7 inches (18 cm) long. Wrap tightly with plastic wrap and refrigerate until firm, at least 3 hours or up to overnight. Line 2 baking sheets with parchment paper. Starting at one end of the log, cut dough into 1/3-inch (9-mm) rounds and transfer to baking sheets.
For Both Types of Cookies: Position 1 rack in upper third and 1 rack in lower third of an oven and preheat to 350°F (180°C). Sprinkle cookies evenly with Demerara sugar and bake until cookies are set at the edges, 10 to 12 minutes, rotating baking sheets halfway through baking. Let cool on baking sheets 5 to 10 minutes, then transfer to wire racks and let cool completely. Makes about forty 2-inch (5-cm) cookies.

Williams Sonoma Gingerbread Cookie Mix