Step 1

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.

Step 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.

Step 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.

Step 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.