Place coconut milk and sugar in a heavy-based saucepan over medium-heat. Whisk until sugar has dissolved. Reduce heat to low and continue to whisk until mixture comes to a simmer. Continue to simmer, not stirring, for 25 minutes. Transfer into a bowl and place in the freezer for 10 minutes to cool.
Meanwhile, prepare the base. Place biscuits in a food processor and blitz until they resemble fine breadcrumbs. Transfer to a mixing bowl and add melted margarine and cocoa. Stir until combined.
Line a 18x28cm slice pan with baking paper. Press base mixture into lined pan and refrigerate for 30 minutes.
Place desiccated coconut, cherries and melted copha in a large mixing bowl. Add cooled coconut milk and a few drops of pink food colouring (if using). Stir to combine.
Remove slice pan from fridge and firmly press coconut mixture on top of the chilled base. Return to refrigerator for a further 20 minutes.
Spread melted chocolate over the slice and return to the refrigerator until set (about 40 minutes). Cut into squares to serve