In a large bowl, combine flank steak, brown sugar, bourbon, and red pepper flakes. Rub the sugar and red pepper into the steak. Transfer the steak and marinade into a 1 gallon plastic bag and let marinate in the refrigerator for 30 minutes or up to 3 hours.
Turn the broiler on high with the top rack 4-6 inches from the flame. Lay the steak on a baking sheet (it will warp, so don't use your best one) and generously season the steak with salt. Place the steak under the broiler and cook for 3 minutes per side for medium rare. Remove the steak from the oven and let rest for 5-10 minutes. Slice into ¼ inch pieces to serve.
Recipe by Things I Made Today at https://www.thingsimadetoday.com/2015/03/30/bourbon-sugar-steak/