Place the garlic in a food processor or mini blender with the chilli, ginger, shrimp paste, peanuts and salt. Blend to a paste. Heat the sunflower oil in a pan and then tip in the garlic paste. Cook for 1 minute, stirring, until the mixture turns golden and smells fragrant. Pour in the coconut milk and simmer for about 5 minutes or until thickened, then tip into a shallow non-metallic dish and leave to cool completely. Stir in the steak and marinate for at least 2 hours (overnight is best, covered with clingfilm in the fridge).
To make the peanut sauce, heat half the sunflower oil in a pan and fry the peanuts for 1–2 minutes until sizzling and lightly golden. Transfer to a food processor and finely chop, then set aside. Place the shallots, garlic, shrimp paste, chilli powder and salt in a pestle and mortar or food processor and blend to a paste.
Add the remaining oil to the pan and tip in the shallot paste. Cook for 30 seconds, stirring, then pour in 300 ml (½ pint) water. Bring to the boil, then stir in the peanut paste, lemon juice and sugar. Reduce the heat and simmer for about 5 minutes until thickened. Remove from the heat and leave to cool completely. The sauce will keep in the fridge, covered with clingfilm, for up to 24 hours.
If using a charcoal barbecue, light it 30 minutes before you want to start cooking. If using a gas barbecue, light it 10 minutes beforehand. Otherwise preheat the grill. Soak twelve 6 inch bamboo skewers in plenty of water for 30 minutes, then drain. Thread the marinated beef on to the skewers and arrange on the grill rack. Cook over medium-hot coals or under a medium grill for 8–10 minutes or until cooked through and lightly charred. Divide the beef skewers between warmed plates, then pour the sauce into dipping bowls and serve alongside.