Prepare the Base: In a medium bowl, combine the crushed digestive biscuits and melted butter until well mixed. Press this mixture firmly into the base of a lined 8x8 inch (20x20 cm) square baking pan. Chill in the fridge for 30 minutes to set.
Make the Caramel Layer: In a medium saucepan, combine the sweetened condensed milk, golden syrup, and brown sugar over medium heat. Stir continuously for about 8-10 minutes until the mixture thickens and turns a golden color. Be careful not to let it boil. Remove from heat and pour the caramel over the chilled biscuit base, spreading evenly. Return to the fridge for another 30 minutes to set.
Prepare the Chocolate Topping: In a small saucepan or microwave, melt the dark chocolate and coconut oil together until smooth. Stir until fully combined. Pour the melted chocolate over the caramel layer, spreading it evenly with a spatula.
Chill and Garnish: Allow the slice to set in the refrigerator for at least 2 hours or until completely firm. Once set, remove from the pan and cut into squares. Sprinkle a light pinch of sea salt flakes on top of each piece for added flavor.
Serve: Place the slices on a decorative plate and enjoy!