Pour sea salt onto a small plate. Rub the rim of two mugs with a slice of lemon and dip the rims into the salt. Scrape out vanilla seeds from half the pod. In a small bowl, mix the softened butter with maple syrup and vanilla bean seeds, and set aside. In a small saucepan, bring the rum and apple cider to a simmer over medium-high heat. Divide the liquid between the two mugs and top each with half of the butter mixture. Serve immediately. Makes 2.