Rinse the lentils. Heat the lentils, liquid of choice, and curry powder to a simmer, stir, cover partially and cook 15 minutes. Stir now and then.
Add the cubed potatoes and butter and stir in, then cover the pan completely and cook 10 minutes or so, depending on how large the potato cubes are. If the lentils start to look too dry, add some more water. You don't want them soupy but they should be moist.
Add salt and pepper to taste and cook another 10 minutes or so. Serve with yogurt and cilantro.