To make the barbecue sauce, place the ketchup, espresso, vinegar, honey, soy sauce, and pepper in a small saucepan.
Whisk to combine and bring to a simmer over medium-low heat. Cook, whisking frequently, for 10 minutes.
Once cooked, divide the sauce into two portions and set both aside.
Place the sliced chicken breast in a large bowl and sprinkle with salt, paprika, garlic powder, and onion powder. Evenly rub the seasonings onto the chicken.
Thread the seasoned chicken, bell peppers, and onions onto bamboo skewers.
Preheat a grill or grill pan over medium-high heat.
Brush one side of the skewers with some of the barbecue sauce. Keep one portion of the barbecue sauce for using with the raw chicken and the other for serving with the cooked chicken.
Place the skewers on the grill and cook for 5 minutes.
Flip the skewers and brush with more barbecue sauce. Continue grilling for 5 to 7 minutes, or until cooked through.
Remove the skewers from the grill.
Sprinkle the skewers with chopped scallions and serve with the reserved barbecue sauce.