Heat the oven to 190C/375F/Gas 5. Line a 30cm/12in square tin with baking parchment.
Melt the butter and the chocolate together very gently.
Mix the cocoa with the flour and the sugar and, off the heat, mix this mixture into the butter and chocolate. Mix in the eggs and fold in the marshmallows.
Turn 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.