Combine the chocolate chips and cream in a large microwave-safe bowl and heat for 2 1/2 minutes. Stir until completely smooth (the chocolate may appear separated at first, but just keep stirring). Pour into the prepared pie crust, loosely cover with plastic wrap and refrigerate until set, about 4 hours or up to overnight.
Preheat the broiler.
Use kitchen shears to cut each marshmallow in half, on a bias, from one corner to the opposite corner. Arrange the marshmallow halves, cut-side down, on top of the chilled pie. Broil until golden, watching closely and rotating the pan as needed, about 1 minute. Let the pie cool completely, then cover loosely with plastic wrap and return to the fridge until the ganache sets again, 1 to 2 hours.
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