First up, preheat your oven to 180°C/ 160°C Fan/Gas Mark 4. Grease and line a 12”x8” rectangle baking tin. Break the Chocolate into pieces and place in a microwavable bowl, add the butter. Melt the chocolate and butter in the microwave in 30 second bursts, stirring between each burst until the mixture is melted.
Next up, pop the sugar into the melted mixture and beat until smooth and the sugar is combined. Add the eggs and Vanilla Extract to the mixture and whisk on a high speed for 2-3 minutes; this is easiest to do with a freestanding or hand whisk, whisking the eggs really well into the mixture helps give the brownie a wonderful gooey texture.
Sieve the flour and Cocoa Powder on top of the mixture and fold through until just combined. Finally pour in the crushed cookies and Chocolate Chunks then mix through. Pour the mixture into the prepared tin and smooth the top. Place in the oven and bake for 25 minutes. Quickly remove the brownie from the oven and scatter the marshmallows over the top of the brownie and bake for a further 5-10 minutes until the edges of the brownie are firm and the marshmallows are lightly toasted and melted. Leave the brownie to cool in the tin.
Slice your brownie into pieces using a warm knife and you’re ready to serve your smores brownie!