Preheat your oven at 180 degree Celsius.
Line your brownie pan with baking paper/wax paper to avoid the brownie from sticking to the pan. Make sure to leave an overhang of baking paper on either sides – this makes it easy to lift the brownies from the pan.
Melt your chocolate with butter in a microwave-safe bowl or over a double boiler* until both the ingredients are fully melted and combined.
Add granulated sugar into the chocolate mix and stir it with a whisk until combined while it’s still hot.* (PS: the sugar will not dissolve completely, so don’t worry if the batter looks grainy at this point)
Once the sugar is combined, add the eggs and vanilla extract and whisk it again until the eggs are well incorporated into the chocolate batter.
Add the cocoa powder, all-purpose flour, salt, chocolate chips, and half of the marshmallows, and digestive biscuits and fold them into the chocolate batter using a spatula or a wooden spoon. This takes some time and energy, so be patient while you’re doing it! There should be no streaks of flour left in the batter by the time you’re done mixing it all in. After the dry ingredients are incorporated, you should end up with a very thick batter.
Transfer your brownie batter into your prepared pan, making sure to spread it across evenly with a spatula. Top the brownie with the remaining marshmallows and biscuits.
Bake it for about 30-40 minutes, until the brownies are cooked but still fudgy.* Once your top looks set and the test toothpick has some crumbs, your brownie is ready. Let the brownies cool completely before slicing.