1. Preheat your oven to 350F (175C) and line a 13x9 inch baking pan with parchment paper. If you leave a little bit of paper hanging out on each side, you'll be able to lift the whole thing out of the pan easily to cut it later.

For the crust

1. Add your graham crackers to a food processor and blitz them to bits. We're looking for fine crumbs here.
2. Add the sugar and salt to the food processor and pulse a few times to combine.
3. Add the melted butter to the processor and pulse until you have a mixture like wet sand.
4. Transfer your crumb mixture to the pan and press it evenly into the bottom. Stash this in the fridge while you get on with the brownie batter.

For the brownies

1. Melt your chopped chocolate in a microwavable dish by heating it for 30 seconds and then giving it a quick stir with a rubber spatula. Repeat this until everything is melted. Set aside to cool down a bit.
2. In a large mixing bowl, beat together butter, sugar, and vanilla with an electric mixer until light and fluffy.
3. Beat the eggs in one at a time until well combined.
4. Turn your mixer down to low and slowly add in the melted chocolate. Make sure you scrape out every last luscious drop.
5. In a separate bowl, whisk together the flour, cocoa powder, and salt.
6. With your mixer on low, slowly add the dry ingredients until just combined.
7. Retrieve your pan with its waiting crust from the fridge. Use your rubber spatula to scoop out dollops of the batter all around the pan until you have all that goodness scraped out. Use an offset spatula (or a butter knife. That's almost as good.) to ever so gently spread the batter evenly over the crust. Try not to disturb your lovely crumbs.
8. Bake the brownies for about 25 minutes. Better underbaked than overbaked, in my opinion.
9. Once you have your brownies out of the oven, switch your broiler on.
10. Dump your mini-marshmallows onto your brownies. You want to create one even, single layer of marshmallows.
11. Pop your pan back in the oven under the broiler. DO NOT walk away. Keep a close eye on your marshmallows as they brown. Once you have achieved peak roastiness, remove your brownies from the oven to cool. Don't worry if there's a bit of smoke. If you've kept your eye on things the whole time, you'll be fine.
12. Let those brownies sit there until they're completely cool. Use a damp, sharp knife to cut them into 15 squares.