Season the Boneless, Skinless Chicken Thigh (4) with Kosher Salt and Freshly Ground Black Pepper (to taste).
Whisk together Soy Sauce (1 tablespoon), Fish Sauce (1 tablespoon), and Apple Cider Vinegar (1 tablespoon) in a large bowl. Place the chicken in the bowl and toss to coat.
Marinate for 20 minutes, then thread chicken onto skewers.
Preheat a grill to medium-high.
To make the peanut sauce, put Unsweetened Coconut Milk (1 can) and Thai Red Curry Paste (2 ounce) into a medium heavy-bottomed pot.
Add Unsweetened Natural Creamy Peanut Butter (3/4 cup), Salt (1/2 tablespoon) and Granulated Sugar (3/4 cup) to the pot.
Add Apple Cider Vinegar (2 tablespoon) and Water (1/2 cup) to the pot.
Let the mixture simmer for 3 to 5 minutes over low heat; be careful not to let the mixture scorch at the bottom of the pot.
Take the pot off the heat, let the sauce cool down to room temperature (or slightly warmer).
Grill the chicken until cooked through, 3-4 minutes total. Serve with the sauce.