Add the coconut milk, caster sugar and agar flakes to a pan and bring to a simmer over a low–medium heat for 4–5 minutes until you can see that all of the agar flakes have fully dissolved.
Pour the mixture into two 180ml dariole moulds, then refrigerate overnight, or for at least 6 hours, to allow the mixture to set completely.
To make the mango coulis, add the pineapple juice and two-thirds of the diced mango to a pan and simmer for 10 minutes until softened and combined. Pour into a high-powered jug blender (or use a hand blender) and blitz until Remove the panna cotta from the fridge and stand the moulds in a bowl of hot water for up to a minute, to make removal easier. Place the panna cotta moulds on serving plates and gently shake to allow the panna cotta to slide onto the plates.
Stir the remaining mango into the coulis, then pour over the panna cotta and serve immediately.