Preheat oven to 350 degrees. Line a rimmed baking sheet with a nonstick baking mat or parchment paper. Place graham crackers, sugared-side down, in a single layer on baking sheet with edges touching; set aside.
Place butter, sugar, and salt in a small saucepan; bring to a boil over medium-high heat, stirring to dissolve sugar and salt. Reduce heat and cook at a rapid simmer until mixture reduces and is bubbly and syrupy, about 2 minutes. Pour over graham crackers, using an offset spatula to spread if necessary.
Transfer to oven and bake until bubbling, about 8 minutes. Remove from oven, let cool 3 to 4 minutes, and sprinkle with chopped chocolate. Let cool completely; refrigerate briefly if necessary to set chocolate. Break into pieces and serve.
Peppermint crunch may be stored in an airtight container for up to 5 days.