In another shallow dish, whisk together the egg and buttermilk. Set aside.
Bread the chicken cubes by first coating it in the egg/milk mixture then coating the chicken with the panko. Put the coated chicken cubes on a plate. Continue until all the chicken have been breaded.
Pop the plate of chicken in the fridge for about 20 minutes. It'll help the breading stick to the chicken.
Preheat oven to 375 °F (191 °C) and line a large baking sheet with foil then place a wire rack on top.
Place the breaded chicken on the wire rack then bake for 20-25 minutes, or until the chicken is cooked through and the coating is lightly toasted brown.
In the meantime, while the chicken is baking, whisk together vinegar, mayo, and jam in a large bowl.
When chicken is done, pour into the bowl with the sauce and toss to coat all pieces with the sauce.