Stir the graham cracker crumbs, pecans, sugar and butter in a bowl until combined. Press the crumb mixture into the bottom of a 9-inch springform pan. Drizzle with the chocolate syrup and freeze until firm, about 15 minutes.
Spread the vanilla ice cream over the chocolate-drizzled crust in a smooth, even layer, packing the ice cream into the pan. Freeze until firm, about 2 hours.
Remove the cake from the freezer and top with the marshmallows. Toast the topping with a kitchen torch or broil about 4 inches away from the heat until the marshmallows are puffed and evenly browned, rotating the pan as needed. Return the cake to the freezer for 15 minutes. Run a knife around the edge of the cake before removing the side of the pan. Serve immediately or freeze for up to 2 days.