Preheat oven to 375F (180C). Grease an 8-inch baking pan and line bottom and sides with wax paper.
Stir brown sugar, coconut and flour together. Add butter and mix until well--combined. Firmly press mixture into prepared pan using the back of a tablespoon.
Bake for 10-12 minutes until lightly-brown and slightly-risen. Make caramel during this time.
Place butter and golden syrup in a medium saucepan over medium heat, stirring occasionally. When butter has melted, add condensed milk. Cook, stirring constantly, for about 9-12 minutes, or until caramel thickens,
Gently spread caramel evenly over base. Return to oven for a further 10-12 minutes (a border of lightly-browned caramel will form around the edges of the slice). Allow slice to cool to room temperature.
For the chocolate topping: Melt chocolate and butter in a small saucepan over VERY low heat. Whisk well to incorporate butter into chocolate and spread evenly over caramel slice. Refrigerate until chocolate is set, about 30 minutes. Holding onto wax paper, remove slice from baking pan. Cut into 12 squares, using a hot, dry knife. You may have to dip your knife into very hot water (and dry) before making subsequent cuts, to keep the cuts neat.