This pie can be made up to two days in advance and refrigerated. When you're ready to serve, sprinkle with flaky sea salt, and if desired, garnish with toasted marshmallows.
Preheat oven to 350 degrees. Finely grind crackers in a food processor (you should have about 1 1/2 cups). Add butter, sugar, and 1/4 teaspoon salt; pulse until combined. Firmly press crumb mixture into bottom and up sides of a 9-inch pie dish. Bake until crust is fragrant and slightly darker, 12 to 14 minutes. Transfer to a wire rack; let cool completely.2.
Place chocolate in a medium heatproof bowl. Bring cream just to a boil in a small saucepan over medium-high. Pour over chocolate; add a pinch of salt. Let stand 10 minutes, then stir until smooth and shiny.3.
Preheat broiler with rack 6 inches from heating element. Brush a rimmed baking sheet with butter; arrange marshmallows in a single layer. Broil until well-toasted and puffed, about 1 minute. With a rubber spatula, gently fold marshmallows into chocolate mixture until just incorporated but still streaky. Scrape mixture into piecrust. Refrigerate until set, at least 2 hours and up to 2 days. Before serving, sprinkle with flaky salt.4.
Optional garnish: Before serving, broil a mound of marshmallows (about a 4-inch circle) on a buttered rimmed baking sheet until well-toasted and puffed, about 1 minute. Using a buttered metal spatula, transfer marshmallows to pie, toasted-sides up.