Instructions

1. Chill the bowls. Depending on your ice cream machine, you may need to chill its bowl in the freezer for 15 to 24 hours before churning the watermelon sherbet. Place 1 (2 1/2-quart) and 1 (1 1/2-quart) mixing bowl in the freezer to chill.
2. Build the rind of the watermelon roll. Remove the 2 mixing bowls from the freezer. Line the inside of the 2 1/2-quart bowl with plastic wrap, leaving several inches of overhang. Paint streaks of green food coloring onto the plastic wrap up the sides and on the bottom of the bowl. Spoon the ice cream into that bowl. Line the outside of the 1 1/2-quart bowl with plastic wrap. Press it inside the large ice cream-filled bowl. The ice cream should form an even 1/2- to 1-inch-thick layer between the bowls. Freeze for 30 minutes or until firm.
3. Make the watermelon sherbet base. Blend the watermelon, sweetened condensed milk, lime juice, and salt in a blender until smooth.
4. Churn the watermelon sherbet. Pour the watermelon sherbet base into the chilled bowl of your ice cream machine and churn to a soft-serve consistency, according to manufacturer's instructions. For many ice cream machines, this process will take about 20 minutes. Add the chocolate chips during the final 2 minutes of churning.
5. Finish building the watermelon roll. Take the ice cream-filled mixing bowl out of the freezer and remove the smaller bowl and plastic wrap. Pour the watermelon sherbet into the hollowed-out area. Cover with plastic wrap and freeze until firm, at least 4 hours.
6. Serve the watermelon roll. Discard the top layer of plastic wrap and invert the bowl onto a serving platter. Remove the bowl and the outer layer of plastic wrap. Cut into wedges and serve immediately.