best pork tenderloin I've ever cooked

The pork tenderloin sounds delicious.
I’ve cooked the butter poached potatoes before and they came out all right. The recipe calls for unsalted butter.
Dumb question here but: do you wait until the butter reaches room temperature, then apply the butter over the potatoes before you add them to the bag? I added slices of still cold butter to the bag, sealed it and noticed after cooking that the slices never really melted around the potatoes.

Thanks!
Vin