Preheat the oven to 350 degrees . Line two muffin pans with baking liners. Chop 3 chocolate bars and place in a small bowl. Chop the remaining 2 chocolate bars and place in another small bowl. In a large saucepan, melt 2 sticks butter in 2 1/4 cups water, stirring often, over medium heat. Remove from the heat and whisk in the 3 chopped chocolate bars until melted. Whisk in the eggs, vanilla and 1/2 teaspoon salt. Whisk in the flour, granulated sugar and baking soda. Divide the batter among the muffin cups, filling each three-quarters full. Bake until the muffins spring back when gently pressed, about 25 minutes. Let cool completely, then turn out of the pans.
In a medium bowl, combine the remaining 2 chopped chocolate bars, 1 1/2 sticks butter and the corn syrup; microwave at medium power for 1 minute. Stir, then microwave 1 minute more at medium power. Stir until smooth.
Dip the top of each cupcake into the chocolate glaze and set right side up on a cooling rack. Dip each cupcake two more times. Let stand until the glaze is set, about 1 hour.
Using an electric mixer, beat the remaining 4 tablespoons butter, the confectioners' sugar and remaining 1/8 teaspoon salt at high speed until fluffy. Add the marshmallow cream and mix at medium speed. Transfer to a pastry bag fitted with a small tip. Decorate the cupcakes with a zigzag design, or dollop with a spoon.