Cut 500g of boneless, skinless chicken thighs into ⅝in / 1.5cm cubes. Thread 4-5 pieces onto each bamboo skewer, soaked in water for 30 minutes.
Mix half of 5 tablespoons of kecap manis with 1 tablespoon of melted unsalted butter and brush over the chicken.
For the sauce, heat 1 tablespoon of oil in a saucepan. Cook 2 minced garlic cloves, 1 finely chopped small onion, and 3 chopped bird's eye chilies for 3 minutes.
Add ½ cup of peanut butter, 1 cup of coconut milk, the remaining kecap manis, ½ tablespoon of soy sauce, and ½ teaspoon of salt. Simmer for 10 minutes, whisking occasionally.
Puree with an immersion blender. Stir in ¼ cup of crushed roasted peanuts and lime juice to taste, and simmer for 2 minutes.
Cook the skewers on a hot grill, turning, until golden and the chicken reaches 165°F / 74°C. Serve with the peanut sauce.
