• Line a 20x20 cm (8x8 inch) pan with parchment paper, leaving an overhang around the edges.

Base Layer

• Add the cereal, chopped Maltesers and crushed biscuits to a large bowl. Give it a few stirs so that they're evenly mixed.
• Add about 1-2 cm of water to a medium saucepan over low heat and place a glass bowl overtop. Ensure that the bottom of the bowl isn't touching the water.
• Add the chopped dark chocolate, sweetened condensed milk and butter chopped into pieces into the glass bowl. Melt the mixture together over low heat with the water underneath just simmering.
• Remove the bowl from the heat and stir until smooth.
• Pour the chocolate over the cereal mixture and stir until it's evenly coated.
• Press the mixture into the bottom of the prepared pan and place in the fridge for about 30 minutes.

Chocolate Topping

• Add the chopped chocolate and butter to a double boiler over low heat and melt until smooth. Or add to a heatproof bowl and microwave on medium power (not high power - which is standard/default on some models) for 45-second intervals, stirring between each.
• Once smooth, pour the melted chocolate over top of the base layer, and smooth the top.
• Place the bars back in the fridge to harden. When the chocolate is about 70-80% hardened (it will still look slightly glossy on top), score the top of the bars with a thin, sharp knife where you plan to cut them. Place an extra Malteser on top of each bar.
• When ready to slice, lift the slice out of the pan using the overhang of the parchment paper. Place on a cutting board and slice using a sharp knife using the tracing marks that you scored earlier.