Preheat your oven to 180゚C or 350゚F.
Line a 8 × 8" square baking dish with parchment so that two opposite ends of the parchment are long. This will help you pull out the brownie slab after baking and cooling.
Crush the digestive biscuits either by hand or in a mixer or food processor until it is a fine powder
Mix in 2 tablespoons of the sugar
Add the melted butter and mix well.
Press this mixture into the bottom of your lined pan as evenly as possible.
Bake this for about 10 minutes.
To make the brownie batter, chop up the butter and the dark chocolate.
Heat in a saucepan or in the microwave until melted.
Whisk in the sugar well.
Add the eggs 1 by 1 and mix well after each egg.
Add the vanilla and whisk well.
Add the flour, the cocoa and the salt.
Whisk until incorporated.
Pour this batter over the baked crust.
Return to the oven and bake for 25 to 28 minutes or until the brownie is done. A toothpick inserted in the center will come out with few moist crumbs.
Spread the marshmallows on top of the brownie immediately. Press the marshmallows gently and slightly into the brownie batter. Return to the oven with the broil option or on the top rack with the top heating element on .
Bake for 2 to 5 minutes until the marshmallows are light brown on the top.
Cool in pan for at least a couple of hours until completely cool before removing and slicing.