Heat the oven to 190 C / Gas 5. Line a 30cm (12 in) square tin with baking parchment.
Bring a pan of water to the boil and place the butter and chocolate in a bowl above the pan. Gently heat until melted.
Mix the cocoa with the flour and sugar and, off the heat, mix this into the chocolate and butter.
Mix in the eggs and fold in the marshmallows.
Spoon the brownie mixture into the tin and bake in the oven until the surface has set but still feels squidgy underneath. This will take about 20 minutes. Allow to cool and slice.