Heat oil over medium heat in a medium skillet. Add onions and garlic. Cook until translucent, add chili paste, curry powder and salt. Stir to combine. Stir in peanut butter, coconut milk, vinegar, brown sugar and 2 tablespoons of warm water. Stir well and bring to a simmer. Let simmer for 1 to 2 minutes, stirring until the sauce thickens and the peanut butter dissolves.
Add 2 tablespoons of peanuts to the sauce and then transfer it to a blender. Blend until smooth, adding 2 to 4 tablespoons of boiling water to help sauce emulsify. Transfer it to a bowl. Let sauce cool to room temperature. Before serving, garnish with the remaining peanuts. The sauce may be made a day ahead and refrigerated in an airtight container. Bring back to room temperature for 1 hour before serving.