Prepare the Base: In a medium mixing bowl, combine the finely crushed digestive biscuits and melted unsalted butter. Stir gently until the crumbs are well-coated with butter. Press this mixture evenly and firmly into the base of a lined 8x8 inch (20x20 cm) square baking pan to ensure a solid base. Place the pan in the refrigerator for 30 minutes to allow it to set and firm up.
Make the Caramel Layer: In a medium-sized saucepan, combine the sweetened condensed milk, golden syrup, and packed brown sugar. Heat the mixture over medium flames, stirring constantly. Continue to stir for about 8 to 10 minutes or until the caramel thickens and develops a rich golden hue. Be attentive to avoid boiling. Once ready, remove it from the heat and pour the warm caramel over the chilled biscuit base, spreading it evenly with a spatula. Return the pan to the fridge for an additional 30 minutes to let the caramel layer set properly.
Prepare the Chocolate Topping: In a small saucepan or microwave-safe bowl, combine the dark chocolate and coconut oil. Heat gently until both ingredients melt and mix together smoothly, stirring continuously until fully incorporated. Once melted, pour the luscious chocolate mixture over the set caramel layer, using a spatula to spread it evenly across the surface.
Chill and Garnish: Allow the slice to chill in the refrigerator for at least 2 hours, or until it feels completely firm to the touch. Once set, carefully remove it from the pan and place it on a cutting board. Use a sharp knife to cut it into elegant squares. Just before serving, sprinkle a light pinch of sea salt flakes on top of each square to enhance the overall flavor profile.
Serve: Arrange the delectable slices on a decorative plate and indulge in this delightful treat with friends or family!