Position a rack in the middle of an oven and preheat to 350°F (180°C). Line 2 baking sheets with parchment paper.
In the bowl of an electric mixer fitted with the flat beater, beat the butter and 1/2 cup (85 g) of the cookie mix on low speed until well combined, about 1 minute.
With the mixer on low speed, add the egg and remaining cookie mix in 3 parts. Beat each addition until just combined.
Roll the dough into twelve 1.5-inch (4 cm) balls (about 45g each). Place the dough balls in a single layer on a tray or plate that can fit in the freezer and chill for 15 minutes.
Once the dough is chilled, place the cookie dough balls on the prepared baking sheets about 2 inches (5 cm) apart, 6 cookies per sheet.
For the best results, bake one sheet at a time until the cookies are slightly browned around the edges, 12 minutes for soft, gooey cookies or 14 minutes for crispy, chewy cookies. Alternately, to bake all the cookies at the same time, position racks in the upper and lower thirds of the oven and bake for 12 to 14 minutes, rotating pans 180 degrees and from top to bottom halfway through baking time.
Transfer the baking sheet (or sheets) to a wire rack and let cool for 5 minutes, then transfer the cookies to the wire rack. Enjoy warm or at room temperature.
Makes 12 cookies.

Williams Sonoma Gluten-Free Chocolate Chip Cookie Mix