For the Marinade: Grind coriander, cumin seeds, and white peppercorns in a mortar and pestle until very fine, then add galangal, lemongrass and fresh turmeric (if using); pound until fine. Add turmeric powder (if using), cinnamon, clove, brown sugar, salt, vinegar, coconut milk, and water; stir to mix well. Pour the marinade over the meat, massage with your hands, making sure that no pieces of meat are stuck together. Cover and refrigerate for at least 2 hours or overnight.
For the peanut sauce: In a food processor, grind the peanuts into a fine meal. If using sesame seeds, grind them into a fine meal using a spice/coffee grinder or a mortar and pestle, being careful not to over grind them into sesame butter. In a small pot over medium heat, reduce 200ml of the coconut milk by about half. Add curry paste and cook over medium low heat, stirring constantly, for about 2 minutes until it is very thick. If it thickens too quickly, add a splash of coconut milk to help loosen it up. Stir in the rest of the coconut milk (save some for basting), ground peanuts, ground sesame seeds (if using), fish sauce, palm sugar and tamarind juice. Simmer for a few more minutes until it has thickened into a dip consistency, stirring frequently to prevent the bottom from scorching. Taste and adjust seasoning with more sugar, fish sauce, and tamarind juice as needed.
To Grill: Preheat the grill to medium. Skewer the meat onto bamboo skewers, about 3 pieces to each skewer, and lay them flat on a tray. Brush the top side of the skewers with coconut milk, then place on the grill, coconut-nut-milk-side-down. Brush the other side with coconut milk and grill for a minute or so just until it is halfway cooked. Flip the skewers and grill the other side just until done. These are small and thin skewers and should only take a couple of minutes to grill, so watch them carefully and do not overcook them!