Cook the elbows to al dente in plenty of salted water according to package directions. Drain, add them back to the pan off the heat and toss with a tablespoon of butter to prevent sticking.
Preheat the oven to 400 degrees F. Butter a 10-inch round casserole with high sides or a 9 x 13.
In a large, heavy-bottomed saucepan melt 3 tablespoons butter over medium-low heat. Stir in the flour and cook 3 minutes, stirring often.
Slowly whisk in the milk and the mustard. Turn the heat up and bring to a simmer. Simmer, stirring often, 4 - 5 minutes.
Turn the heat down to low and add both cheeses except for ½ cup for the top, stir until completely melted and smooth. Remove from heat and season with 1 teaspoon salt and ½ teaspoon black pepper.
Stir the black beans into the pasta. If the pan is big enough add the pasta to the cheese sauce or vice versa, mix gently with a wooden spoon and pour into the casserole dish. (If your pan(s) aren't big enough add half the pasta to the casserole dish and pour half the cheese sauce over it. Add the remaining pasta and pour the rest of the sauce over it.)
Sprinkle with the remaining cheese, crushed chips and dot the top with the cut up butter.
Bake 25 minutes until bubbly and golden on top. Let it sit 5 - 10 minutes before serving.