Preheat your oven to 350°F / 180°C (160° fan-assisted) and line a 9-inch square pan with parchment paper.

Blitz the graham crackers or digestive biscuits in a food processor until they are crumbs.

Melt the butter in the microwave in 30-second increments until melted, pour into the graham cracker crumbs, and mix together until well combined.

Tip the crumb mixture out into the prepared pan and use your hands or a glass to press the crumbs down into a tightly packed, even layer.

Chop the chocolate into small pieces and put it into a heatproof bowl along with the butter. Melt in the microwave in 30-second increments, stirring in between, until totally melted. Set aside.

In a large bowl, combine the sugar and eggs and whisk until thick and custard-like.

Pour the melted chocolate mixture into the egg/sugar mixture, add the vanilla, and gently whisk everything together until fully incorporated.

Sift the flour and salt into the bowl and use a spoon or spatula to fold everything together until there are no more visible lumps of flour.

Pour the brownie batter into the cake pan on top of the crumb base.

Bake for 25 - 30 minutes for a soft, gooey brownie, or 30 - 35 minutes for a dense, sturdy brownie.

Let the brownies cool in the pan completely before attempting to unmold them.

Make a double boiler by placing a heatproof bowl over a small saucepan with a small amount of water in it.

Place the egg whites, sugar, and cream of tartar in the bowl and put the saucepan on medium-low heat to bring the water to a simmer.

Whisk constantly to bring everything together until the sugar has dissolved.

Once the eggs are warmed and the sugar has dissolved, remove the bowl from the heat, add the vanilla, and start whipping on high speed.

Whip the meringue until it is thick and glossy.

Once the brownies have completely cooled, unmold them from the pan.

Slather the marshmallow frosting all over the top of the brownies and spread it out to the edges.

Using a kitchen torch, carefully toast the marshmallow frosting to your desired toastiness.

Slice the brownies into 9 or 16 squares and enjoy!