Line your square pan with parchment paper to make removal easy, then set aside.
Crush the digestive biscuits into small, uneven crumbs using a rolling pin or pulse in a food processor—aim for about 2 cups of crushed crumbs. This gives the bars their sturdy base.
Chop the dark chocolate into small pieces and set aside. Place it in a heatproof bowl.
Set a saucepan with a couple of inches of water over low heat and bring to a gentle simmer. Place the bowl with chocolate on top, ensuring the water doesn't touch the bowl's bottom.
Stir the chocolate constantly as it melts, until it’s smooth, shiny, and completely melted—this takes about 3-5 minutes. Remove from heat once glossy.
Stir in the butter and sifted cocoa powder into the melted chocolate, blending until the mixture is glossy and smooth. This adds richness and depth to the chocolate.
In a large mixing bowl, combine the crushed biscuits with the sweetened condensed milk and vanilla extract. Mix until evenly coated and sticky.
Pour the warm, melted chocolate mixture over the crushed biscuits and stir thoroughly, ensuring every crumb is coated in glossy chocolate. The mixture should be thick and cohesive.
Transfer the mixture into the prepared pan. Use a spatula to press it down firmly and spread evenly, creating a smooth top. Pressing helps eliminate air pockets and ensures clean slices.
Cover the pan with plastic wrap or foil and refrigerate for at least 2 hours, or overnight, until the mixture is firm and shiny.
Once set, lift the bars out of the pan using the parchment paper. Use a sharp knife dipped in hot water to cut clean, even squares.
Serve immediately or store in an airtight container in the fridge for up to 5 days, enjoying their fudgy, crumbly texture and rich chocolate flavor.
