Blend milk and 1 pint ice cream in a blender until smooth. With machine running, add remaining pint ice cream, one scoop at a time; blend until smooth. Pour milkshake into six 8-ounce glasses decorated with chocolate faces, filling them 3/4 full.
Put cream in the bowl of an electric mixer fitted with the whisk attachment. Beat on medium speed until soft peaks form. Add confectioners' sugar. Beat until stiff peaks form.Transfer whipped cream to a pastry bag fitted with a 1/2-inch plain round tip (such as Ateco #806). Pipe spiral mounds of whipped cream on top of milkshakes. Serve immediately.
Tip: This recipe can be doubled easily. For 12 faces, place 1/3 cup (2 ounces) semisweet chocolate chips in a microwave-safe bowl; microwave on high until melted, 1 to 1 1/2 minutes; stir. Using a small brush, paint chocolate inside glasses (if it hardens too soon, microwave 20 seconds more). Let set a few minutes before adding shake. Prepare glasses up to a day ahead, but don't fill until ready to serve.