Line a 9-by-13-inch pan with foil. Heat the marshmallows, sugar, cream, butter and salt in a medium saucepan set over medium heat, stirring constantly, until the marshmallows melt and the sugar dissolves. Remove the saucepan from the heat, add the white chocolate chips and vanilla, and stir until the chips are completely melted.
Pour the mixture into the prepared pan, and scatter the chocolate chips over the top. Lightly press the chips into the fudge. Let cool to room temperature, then cover with plastic wrap and refrigerate until firm, about 1 hour. Cut into 1-inch squares.
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