Fill a medium saucepot with 1 inch of water and place over medium-low heat to bring the water to a simmer. Place the bittersweet and Mexican chocolate in a bowl that will fit over the top of the saucepot without touching the water. Melt the chocolate over the water, stirring occasionally, until smooth.
Place the cream and salt in a saucepot over medium-high heat and bring to a bare simmer. Slowly whisk the warm cream, 1/2 cup at a time, into the chocolate until the dip has the consistency of chocolate sauce. Transfer to a serving bowl or fondue pot and cover to keep warm until ready to use. Serve with the marshmallows, sprinkles, nuts and fruit.