Penne with Pesto, Potatoes and Green Beans

1 1/2 cups lightly packed fresh basil leaves 

3 Tbs. pine nuts 

1 garlic clove, coarsely chopped 

1/2 tsp. salt, plus more, to taste 

1/3 cup extra-virgin olive oil 

1/4 lb. Parmigiano-Reggiano cheese, grated  

2 Tbs. salt 

1/2 lb. small red potatoes, peeled and cut into slices about 1/4 inch thick 

1/2 lb. young slender green beans, stem ends trimmed 

1 lb. penne, ziti or trenette  

1 Tbs. unsalted butter, at room temperature