Place the 2 lb boneless skinless chicken breasts in the insert of a 6-qt slow cooker in a single layer.
In a mixing bowl, whisk together 2 to 4 Tbsp cayenne pepper (start with 2), 3 Tbsp brown sugar, 1 tsp garlic powder, 1 tsp paprika, 1 tsp chili powder, melted 1/4 cup butter, and 3/4 cup chicken broth until combined.
Pour the spice-butter mixture evenly over the chicken in the slow cooker.
Cover and cook on LOW for 4 to 6 hours, until the chicken is cooked through and tender.
Remove the cooked chicken to a cutting board or bowl and shred with two forks.
Return the shredded chicken to the slow cooker and stir to coat in the sauce; taste and add more cayenne if desired.
Serve the Nashville hot chicken on buns with pickles and ranch, as desired.
