Preheat oven to 350°F. Bring the cream to a simmer in a small saucepan, and put the chocolate in a bowl. Pour the cream over the chocolate and let sit a few minutes, then whisk until smooth. Whisk in the egg and salt. Pour into the crust.
Bake for 25 minutes, until filling is softly set but still trembles slightly in the middle when you gently shake it. If it starts to darken around the edges, loosely cover the edge of the pie with a pie shield or foil. Cool completely on a wire rack. (Keep the pie shield or foil in place - it'll come in handy when it goes under the broiler.)
Pile the marshmallows on top of the cooled filling and preheat the broiler. Broil the pie for a minute - it browns quickly, so check it after 30 seconds - until the marshmallows are golden brown. Cool for 10 minutes before slicing with a sharp knife dipped in hot water, then wiped dry (do this before you cut each slice).