Combine the cashews and softened butter in a food processor and pulse to a smooth paste. Or, finely chop the cashews and combine them with the butter.
In a medium, non-stick frying pan, heat the oil over moderate heat. Season the chicken breasts with half of the salt and pepper and add them to the pan. Cook until browned and just done ~ four - five minutes per side. Remove the chicken from the pan and let is rest for five minutes. Cut into small pieces.
In a large pot of boiling, salted water, cook the penne until just done ~ about 13 minutes. Add the beans during the last five minutes of cooking. Drain the pasta and beans and toss with the cashew butter, chicken and the remaining salt and pepper.
Cashews taste delicious in this dish but you could really use almost any kind of nuts such as almonds, hazelnuts, peanuts, pecans, pistachios or walnuts.