Line your 20x20cm (8x8 inch) baking dish with parchment paper, leaving some overhang for easy removal.
Break the digestive biscuits into small chunks, aiming for a mix of fine crumbs and some larger pieces for texture.
In a heatproof bowl, melt the butter gently over a double boiler or in short bursts in the microwave, stirring until smooth. You should hear a gentle bubbling or crackling, and see the butter turn glossy.
Add the cocoa powder and condensed milk to the melted butter, then stir until the mixture is smooth, shiny, and well combined. It will smell rich and chocolatey.
Pour the chocolate mixture over the biscuit chunks and gently fold with a spatula until all pieces are coated and the mixture is thick and sticky.
Transfer the mixture into your prepared dish. Use the back of a spoon or spatula to press it firmly into an even layer, making sure to eliminate any air pockets. Tap the dish gently on the counter to settle everything.
In a separate bowl, melt the dark chocolate slowly over simmering water or in short microwave bursts, stirring until glossy and smooth. Pour this over the pressed biscuit layer and spread evenly with a spatula.
Place the dish in the fridge and chill for at least 3 hours, or overnight if possible, until the chocolate topping is set and firm to the touch.
Once set, lift the entire block out of the dish using the parchment overhang. Use a sharp knife to cut into 12 evenly sized squares, wiping the blade clean between cuts for neat edges.
Serve immediately or store in an airtight container in the fridge for up to a week. For longer storage, wrap tightly and freeze. Enjoy these crunchy, chewy chocolate treats anytime!
