In a large bowl, whisk together the brown sugar, smoked paprika, garlic powder, onion powder, kosher salt, and black pepper to make the spice blend.
Trim any fat or gristle from the chicken breasts and cut into 1-inch cubes.
Add the diced chicken to the bowl with the spice blend and toss thoroughly to coat each piece evenly.
Heat the olive oil in a large skillet over medium-high heat until shimmering, about 1 minute, then reduce heat to medium.
Arrange the chicken pieces in a single layer and cook undisturbed until the bottoms are golden brown, about 3 minutes.
Flip the pieces and cook for about 2 more minutes, until nearly cooked through.
Add the butter to the skillet and stir or toss the chicken for 1–3 minutes until the butter melts and the chicken reaches an internal temperature of 165°F; remove from heat.
Garnish with chopped parsley if desired and serve immediately.