Place the chicken breasts in a large ziploc plastic bag. Pour in the buttermilk to cover the chicken. Add the rosemary sprigs. Seal the bag and refrigerate overnight, about 8-10 hours.
Preheat a grill to medium-high heat. With tongs, take out each chicken breast one by one, shaking off excess buttermilk and removing the rosemary sprigs.
Grill for 4-6 minutes per side, until the chicken is cooked through but still moist. Tent with foil for about 5-10 minutes before slicing and serving.
Notes
Buttermilk: here is a guide for making your own buttermilk. Quantity: this recipe is so flexible and easy you can double and triple to your heart’s delight. The idea is basic – just add enough buttermilk to cover the chicken and throw in as much rosemary as your soul can take. I like a lot of rosemary flavor. Use what you like.