Step 1: Line a 20cm x 20cm (8x8 inch) square baking tin with parchment paper, allowing an overhang on two sides for easy removal.
Step 2: In a large bowl, combine the finely crushed digestive biscuits with the melted butter and sweetened condensed milk. Mix well until everything is thoroughly combined and resembles wet sand.
Step 3: Press the biscuit mixture firmly and evenly into the prepared baking tin. Use the back of a spoon or your knuckles to ensure a compact base. Place the tin in the refrigerator to chill for at least 30 minutes while you prepare the topping.
Step 4: For the chocolate topping, melt the chopped dark or semi-sweet chocolate in a heatproof bowl set over a saucepan of simmering water (bain-marie method) or in the microwave in 30-second intervals, stirring until smooth. If the chocolate is too thick, stir in 1-2 tablespoons of milk until it reaches a pourable consistency.
Step 5: Stir 1 cup of shredded coconut into the melted chocolate.
Step 6: Pour the chocolate-coconut mixture evenly over the chilled biscuit base. Spread with an offset spatula or the back of a spoon to create a smooth layer. If desired, sprinkle with extra shredded coconut for garnish.
Step 7: Return the tin to the refrigerator and chill for at least 2 hours, or until the chocolate topping is completely set. For best results, chill overnight.
Step 8: Once set, use the parchment paper overhang to lift the slice from the tin. Place on a cutting board and cut into squares or rectangles using a sharp, hot knife (run under hot water and wipe dry between cuts for clean slices). Serve and enjoy!