Line a 20cmx20cm baking tray with baking parchment.
Place the bananas in a large bowl with the lemon juice, and mash with a fork until the banana turns to puree. Stir in the dessicated coconut and pineapple chunks. Spoon the mixture into the baking tray and squash down firmly - right to the edges. Use the back of a spoon to ensure the mixture is completely flat. Cover with clingfilm and freezer for 3 hours (or your can freeze overnight if you wish)
Melt the chocolate in a medium sized bowl. You can do this either in 30 second bursts in the microwave - stirring well between each burst - or over a pan of simmering water on the cooker (make sure the bowl isn't touching the water though).
Once the chocolate has melted, take the coconut mixture out of the freezer. Place on a chopping board and remove the tin and the baking parchment. Slice into 12 bars using a sharp knife.
Take one of the bars (put the rest back in the freezer whilst you're working to keep them solid) and rest it on a fork, then dip the bottom of the bar in the melted chocolate. Use a spoon to spoon melted chocolate all over the top and sides. Use the spoon to scoop any dripping chocolate from the bottom of the bar (whilst it's resting on the fork) and place the bar on some baking parchment to set.
Repeat with the remaining bars and serve once the chocolate has set.