Put some whipped cream in the bottom of a tall glass, then add a drizzle of chocolate syrup or chocolate sauce. About 1/4 cup whipped cream and a tablespoon of chocolate sauce.
Blend on medium speed until you no longer hear chunks of ice hitting the sides of the blender, then blend on high for 20 seconds or so, then back to medium for about 10 seconds, then low for another 10 seconds or so.