Chop the butter and chocolate up into small pieces and put in a bowl. Microwave on high for 1-2 minutes until melted. Allow to cool.
Whisk the caster sugar and eggs together for 3-5 minutes using an electric mixer, until it looks thick and creamy.
Pour the cooled chocolate and butter mixture over the egg and sugar mixture and fold it in gently until completely mixed.
Sieve in the cocoa powder and flour and again fold in gently until completely incorporated, but do not overmix.
Grease and line your slow cooker bowl with grease proof paper.
Pour the brownie batter in.
Add about 6 of the marshmallows, in pieces, on top.
Cook on high for 1.5 to 2 hours or until just set in the centre. See my note below regarding cooking time.
Half way through, add the rest of the marshmallows on top (the others will have sunk/melted into the brownies).
Allow to cool completely before using the baking paper to remove from the slow cooker.