Start by crushing the plain biscuits in a food processor until they resemble fine crumbs.

In a mixing bowl, combine the crushed biscuits, sweetened condensed milk, cocoa powder, and icing sugar. Stir until you have a thick mixture that holds its shape.

Line a rectangular dish or a baking tray with parchment paper. Press half of your biscuit mixture evenly into the bottom of the dish. Use a spatula to smooth it down.

Place the dish in the fridge for about 15-20 minutes, allowing the base to firm up.

While the base is chilling, melt the dark chocolate and unsalted butter together in a double boiler or microwave. Mix until smooth.

Once the base is firm, spread the remaining biscuit mixture on top, pressing it down gently to create an even layer.

Return to the fridge for another 15-20 minutes to set.

After the layers have set, remove the dish, cut the mixture into rectangles, and dip each piece into the melted chocolate mixture. Allow any excess chocolate to drip off.

Place the coated pieces back onto the parchment paper and let them cool until the chocolate has completely set.