Cook the sweet potatoes in lightly salted boiling water for about 20 minutes or until tender. Drain, then return them to the pan and place over a low heat for 1 minute, stirring constantly, so the excess moisture evaporates. Lightly mash the potatoes with a fork.
Meanwhile, put the spinach in a colander and pour over a kettle of boiling water. Refresh the spinach in cold water and squeeze dry. Stir the wilted spinach into the potatoes, then add the spring onions. Season well and set aside.
Make the dip. Gently warm the coconut cream in a pan with the chillies, lemon grass and lime leaves for about 10 minutes. Don't let it boil. Set aside to infuse.
Heat the oil in a large pan or deep-fat fryer to 180°C (350°F) or until a cube of bread browns in 20 seconds. Use your hands to form the potato mixture into 12 cakes. Mix together the sesame seeds and flour and sprinkle over the cakes, then carefully lower them into the oil and fry in batches for about 3 minutes until they are golden and crispy. Drain on kitchen paper and keep warm while you cook the rest.
Stir the coriander and sesame oil into the dip and pour it into 4 individual dishes. Serve immediately with the potato cakes, garnished with lime wedges and spring onions.