Place the chicken and buttermilk in a shallow baking dish and stir around a bit to coat all of the chicken with the buttermilk. Cover and place in the fridge and let marinate for at least 1 hour.
Preheat the oven to 375F and adjust the rack to the middle position. Combine the arrowroot, ground plantain chips, onion powder, sea salt and sesame seeds in a pie plate. Place a piece of marinated chicken in the plantain mixture and press on both sides to coat the chicken. Place the chicken on a baking sheet lined with parchment paper. Repeat with remaining chicken. Combine the melted palm shortening and butter in a small bowl. Using a pastry brush, carefully brush each piece of chicken with the butter mixture. Bake for 25 minutes, or until coating is turning golden brown.
Stir the plum jam, horseradish and lemon juice until combined. Serve the plum sauce with the chicken.
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