Preheat your smoker to 225 degrees F
Slice the cube of butter into thin slivers. You should get around 10 slices.
Slide about half of the slivers of butter under the skin of the turkey. To do this you can carefully slide your hands under the skin to separate the skin from the meat. Don't completely pull the skin off. Just slide your hands under the skin to create space for the butter.
Season the entire bird with a generous layer of your poultry rub
Place the turkey directly on the cooking grate, and let it smoke until the breast reaches an internal temperature of about 120-125 degrees F.
Increase the temperature of the smoker to 375 degrees F
Put the turkey into a foil pan, and place 2-3 slivers of butter under the turkey. Place the remaining slivers of butter on top of the turkey, and lightly sprinkle with a bit more of your poultry rub.
Baste the turkey with a basting brush, using the melted butter and drippings in the foil pan every 15 minutes until the turkey is done.
Let the turkey come up to temperature at around 160-162 degrees F in the breast. Check that the thighs are up to temp as well. They will most likely be closer to 175-185 degrees F. That is completely normal with the thighs, as the fats and collagens in the dark meat start to melt away and become tender at those temperatures. Don't let the breast get above 165 degrees F or it will dry out.
Remove the bird from the smoker, and let it rest on a cookie sheet under a tent of foil for about 20-25 minutes.
Slice and enjoy!