Cool them thoroughly first, then wrap.
Do not wrap them right out of the oven or they will continue to cook from the residual heat.
Also, as I've mentioned before, do not go by the listed baking time, which is too long - bake them about 18-19 minutes. The result will be very moist, deep chocolatey muffins.
Also, I've used both the volume (spoon and sweep), and the weight measurements, and while both methods yielded very good results, the weight measurements yielded superior results, ie, a lighter, more tender texture. I now only use the weight measurements.
And lastly, as in all baking, if you use a dark nonstick pan, decrease the oven temperature by 25 degrees.
If you make these, let me know how they turn out!