Grease and line a 9" square baking dish with parchment paper or cling film.
In a saucepan, over a medium heat, melt together the butter and marshmallows, stirring occasionally until smooth.
Remove from the heat and add the cocoa powder, followed by ¼ cup of the chocolate chips. Stir through to create a chocolate marshmallow sauce.
Either add the rice krispies to the pan or pour this marshmallow mixture over your rice krispies and stir to incorporate. You may need to use your hands to help mix, but do this carefully as the marshmallow will be hot.
Press this gently, but evenly, into your prepared lined baking tray and top with the remaining chocolate chips and a sprinkle of flaked sea salt (optional).
Allow to cool completely (about an hour) before slicing.