Mix all ingredients for the quesadilla sauce in a small bowl. Set aside.

Heat a large (12") skillet over medium heat. Add oil, chicken, and black beans. Sprinkle with 1/8 tsp each cumin and chili powder. Stir to combine. Cook until heated through stirring frequently. About 5 minutes. Set chicken mixture aside. Wipe skillet clean. Return to heat.

Brush one side of one flour tortilla with 1/4 of the melted butter. Place in skillet buttered side down.

Sprinkle half cup of shredded cheese over one half of the tortilla. Tear one slice American cheese into pieces, scatter over shredded cheese.

Top cheese with 1/4 of the chicken mixture. Spread 1/4 of the quesadilla sauce on the other half of the tortilla.

Cook open-faced until cheese is mostly melted. Carefully fold sauce half of tortilla over chicken/cheese half. Press together with spatula. Continue cooking 2 minutes.

Carefully flip quesadilla. Cook 2 minutes more on second side. Repeat process with remaining quesadillas.

Keep finished quesadillas warm while cooking the others by placing on a cookie sheet in a 200°F oven. Cut and serve alone or with additional toppings.