Combine the puffed rice and desiccated coconut in a large bowl.
Place the cocoa powder, coconut oil and Nutella in a heatproof bowl over a saucepan of simmering water (don’t let the bowl touch the water). Stir until the Nutella melts and the mixture is smooth.
Pour the Nutella mixture over the puffed rice mixture and use a spatula to stir until evenly combined. Transfer two-thirds of the puffed rice mixture to a round 26cm (base measurement) fluted tart tin with removable base, pressing gently but firmly into the side and edge of the tin. Pile the remaining puffed rice mixture over the top and press down gently to compact.
Place in the fridge for at least 6 hours or until completely set (it may look like it is set sooner, but it takes at least 6 hours for the centre of the crackle to set).
For the Nutella topping, place the Nutella and coconut oil in a heatproof bowl over a saucepan of simmering water (don’t let the bowl touch the water). Stir until the Nutella melts and the mixture is smooth. Pour the topping over the crackle cake. Sprinkle with coconut flakes.