Combine crushed chocolate wafers and butter in a medium bowl, stirring until it has the consistency of coarse crumbs. Transfer to a fluted tart pan (or regular pie plate); press crumbs into the bottom and up the sides using your hands and/or a flat-bottomed measuring cup.

Combine cream, sugar and corn syrup in a medium saucepan over medium heat. Cook, stirring occasionally, until sugar dissolves and it comes to a bare simmer. Remove from heat and stir in chocolate; whisk until smooth. Stir in vanilla and butter.

Pour 1 cup of fudge sauce into bottom of crust and swirl the pan to coat evenly. Sprinkle with 2/3 cup mini marshmallows and 1/4 cup salted peanuts. Transfer crust to freezer until the the fudge sauce has firmed up, about 30 minutes.

Soften ice cream a bit by removing from freezer for about 10 minutes; spread over fudge layer. Drizzle with more fudge sauce (you may need to microwave a bit to make it pourable, but not hot), then sprinkle with remaining peanuts and marshmallows. Freeze until firm. Reserve remaining fudge sauce for another use (hello, sundaes!).