Rinse the prawns under cold running tap water and let them drain. Then peel and devein them. Put the prawns in a large bowl or baking dish. Sprinkle with the brown sugar, the juice of 2 limes, the cumin seeds, mashed garlic and chipotle powder. Season with a pinch of pepper and salt.
Stir the prawns well and let them marinate in the fridge for at least 1 hour. Stir them regularly. In the meantime peel the cucumber and the mango. Slice the cucumber lengthwise in half and remove the seeds inside with a teaspoon). Then slice the cucumber in little strips and add them to a large mixing bowl. Slice the mango, remove the pit in the middle and slice the flesh into little cubes. Add them to the cucumber together with the freshly chopped cilantro, coriander seeds and chopped red onion.
Grill them under a hot grill for 3 minutes until pink. Then flip them over and grill them for another 3 minutes or until they are cooked through.