Avoid "light" and inexpensive ice cream. It's often whipped with air, which means you'll need more scoops of ice cream—and your shakes will melt faster. The ice cream carton should feel heavy for its size.
Although there's a bit of ice cream in this recipe, the ice makes it lighter and less rich than a typical milkshake. To keep the syrup visible on the sides of the glass, put the glass in the freezer up to 30 minutes before adding the syrup.