Preheat the Oven to 160 degrees Celsius. Grease a 9 x 9 inch square tin and line with tin foil.
Combine the crushed digestives and sugar with the melted butter in a large bowl. Flatten into the lined tin and bake for 10 minutes.
Let the base cool, but leave the oven on. Make the brownie layer by melting together the butter and the Extra dark chocolate in a microwave proof bowl for 30 second bursts in the microwave. Once smooth, set aside.
Whisk the eggs and sugar together until pale and fluffy, for approximately 3 minutes in an electric mixer. Sieve in the dry ingredients, adding the butter and chocolate mixture and fold. Lastly add the chocolate chips and pour over the biscuit base.
Bake for approximately 25-30 minutes depending on how gooey you like your brownies.
Turn off the fan oven and turn on the grill to high. Remove from the oven and scatter over the chocolate chips. Top with plenty of marshmallows and place under the grill, keeping a close eye, until the marshmallows are bubbling and golden.
Let set for at least 1 hour, slice and enjoy!