Position a rack in the middle of an oven and preheat to 350°F (180°C). Grease an 8-inch (20-cm) square baking pan and line with parchment, leaving about a 1-inch (2.5-cm) overhang. Set aside.
To make the crust, put the butter cubes and crust mix in the bowl of an electric mixer fitted with the flat beater. Beat on low speed until the mixture resembles coarse crumbs, 2 to 3 minutes. Increase the speed to medium-high, add the water and beat until the dough starts to come together, about 1 minute. The mixture will be loose.
Transfer the mixture to the prepared pan and press firmly into the bottom. Set aside.
To make the filling, put the cream cheese, eggs and melted butter in the clean bowl of an electric mixer fitted with the flat beater. Beat on medium speed until well combined, 2 to 3 minutes, scraping down the sides of the bowl as needed. Reduce the speed to low, add the filling mix and beat until incorporated, about 1 minute. With the mixer running, add the water and beat until combined, about 30 seconds. Increase the mixer speed to medium and beat for 1 minute more.
Evenly spread the filling over the crust. Bake until the edges are set and deep golden brown, 33 to 36 minutes.
Transfer the pan to a wire rack and let the bars cool in the pan for 45 minutes. Run an offset spatula around the edges of the bars. Remove the bars from the pan using the parchment paper and place on a cutting board. Cut into squares and serve.
Makes sixteen 2-inch (5-cm) bars.

Williams Sonoma Gooey Apple Bar Baking Mix