Preheat the oven to 350 degrees F. Butter a 9-inch springform pan and line the bottom with a parchment paper circle.
Put the melted butter and wafer cookies in a medium bowl and mix well. Press the mixture into the bottom and slightly up the sides of the prepared pan. Bake for 10 minutes, then let cool completely before using.
For the ice cream cake:
Spread the marshmallow creme onto the cookie crust then sprinkle with the walnuts. Scoop the ice cream onto the walnut layer, packing it in tightly. Spread with an offset spatula to make a smooth top. Cover with plastic wrap and freeze until firm, 2 to 2 1/2 hours.
Unmold the cake by running a knife around the edge of it, then unhinge the springform and remove. Invert the cake onto a sheet of plastic wrap and remove the bottom of the pan and the parchment. Place a plate on the bottom and invert once more.
For the chocolate ganache:
Bring the heavy cream to a simmer in a small pot. Pour the cream over the chocolate in a heatproof bowl and stir until smooth. Allow to cool 10 minutes before using.
Frost the top and sides of the cake with the ganache and sprinkle with the mini marshmallows. If the cake begins to get too soft, put it back into the freezer for a few minutes.
Freeze another 1 1/2 to 2 hours or overnight. Before serving, let the cake sit at room temperature for about 20 minutes.
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