The night before, place the brown rice and apple cider vinegar in a large mixing bowl and cover with water (to reduce the arsenic, as mentioned in the post above, use five parts water to one part rice). Leave on the counter overnight. The rice can soak for up to 24 hours.
Before cooking, drain the rice. Place 3 tablespoons of butter in a large pot and melt over medium heat. When the butter is melted, add the drained rice and cook, stirring frequently, until the rice is just slightly toasted, about 3-4 minutes. Add the broth and salt and stir. Increase the heat to high and bring to a boil. As soon as the mixture reaches a boil, put the lid on and reduce the heat to low. Cook for 45 minutes or until rice is cooked. Add the remaining 3 tablespoons butter and stir until melted. Season to taste with salt. Serve.