Combine the chocolate and cream in a large bowl, and microwave gently until melted. Stir every 30 seconds or so then return and heat until chocolate is nearly completely melted. Stir to combine and smooth out.
Pour into prepared pie crust and refrigerate until set for 4 hours or overnight. Chocolate should be set and a little firm to the touch.
Preheat the oven to broil, and scatter the mini marshmallows evenly on the top of the ganache. Broil for 1-2 minutes, rotating halfway through to evenly toast the marshmallows until puffy and golden brown in places.
Carefully remove the pie from the oven and chill in the fridge again until the chocolate re-sets about 1-2 hours. Slice and enjoy!