Bring 1 1/2 pounds new potatoes to a boil in a pot of salted water. Cook until tender, about 20 minutes; drain. Melt 3 tablespoons butter and 1/4 cup milk in the same pot; add 3 tablespoons pesto and the potatoes. Lightly smash with a potato masher. Top with fresh ricotta or sour cream.
Photograph by Andrew Purcell