Preheat the smoker to 225°F and set it for indirect heat.
Prepare the pastrami rub and set it aside.
Remove the turkey breast from the brine, pat it dry, and discard the brine.
Coat the turkey in a thin layer of mayo, then evenly apply the pastrami rub.
Tuck the tapered end of the breast under the thicker section to form a more uniform shape, then secure it with butcher’s twine, knotting the twine under the turkey with the seam side down.
Place the turkey breast onto the grill grates and smoke for an hour.
After the first hour, lightly spray the turkey with chicken stock to enhance bark formation and deepen the smoky flavor.
Continue smoking, spritzing every 30-40 minutes, until the internal temperature reaches 145-150°F.
Increase the grill temperature to 425°F and continue cooking until the outside is golden brown and the thickest part of the breast reaches 165°F.
Baste the turkey with melted butter for added richness.
Remove the turkey from the grill and place it skin-side up on a cutting board.
Allow it to rest for 10-15 minutes before slicing.
Cut against the grain into ¼-inch thick portions and serve warm.