If You Already Own a Blendtec
You will notice that all my dessert smoothie tutorials are using a Vitamix. This is because the carafe/jar has a tamper in it that allows you to move/jostle the material around while it's blending, which I have found is pretty much required in order to get these smoothies to have the ice-cream-like thickness we're going for.
If you have a Blendtec, FEAR NOT my friends!
There are 2 solutions.
NOT IDEAL, BUT WORKS
Using the Blendtec you have, add additional liquid and sweetener as necessary to get it to blend and/or stop the machine periodically to scrape down the sides with a spatula, move the material around/back to the center of the jar. Not as ideal but requires no additional cost.
IDEAL - WORKS AMAZING
Blendtec now has a special new jar you can buy separately called the TWISTER JAR has a top with long stiff plastic sides that lets you move the material around in the blender WHILE it's going, as well as it's smaller than the normal jar which is also better since we make single serving size smoothies.
I have this twister jar myself and have now made at least 100 dessert smoothies in it. I've actually found that it can make even thicker smoothies than the Vitamix, despite Vitamix being my first love. So basically, it works GREAT for these smoothies.
You can buy the Twister Jar here:
I am not sure if this jar fits on all blendtec bases, so you might want to double check that first before you fork out $100+.