Melt the butter and sugar in a medium skillet over medium heat. Stir until the sugar dissolves, about 3 minutes. Increase the heat to high. Add the pineapple and cook, stirring occasionally, until golden and coated in caramel, about 5 minutes. Remove the skillet from the heat and add the vanilla and salt. Divide the pineapple sauce among 4 sundae dishes. Top each dish with 1 scoop of ice cream and some chocolate sauce.