Bring water to a boil over medium-high heat. Add potatoes and cook for 15-25 minutes or until soft enough to pierce all the way through with a fork. Do not overcook. Potatoes shouldn't fall apart when pierced.
Strain out water.
Over low heat, mash and mix potatoes with butter, milk, cream, salt, and pepper. Set aside. Taste for salt and adjust if necessary. Mixture should be flavorful and slightly salty.
Next, prepare a frying pan with oil over medium-high heat. In the meantime, warm tortillas 4 at a time, over a comal (flat griddle) or wrap them in a clean dish towel, and heat in the microwave for approximately 30-45 seconds or until they become soft and malleable.
Working in batches, add potato mixture to tortillas, fold in half, and carefully place tacos in the hot oil in the frying pan. Oil should be very hot and tortilla should sizzle when it touches the oil. You may need to hold the top side of the taco down with tongs for a a few seconds so that it does not unfold. In about 1 to 2 minutes, check underside of taco. If golden brown, turn and fry the other side until golden brown.
Once the taco is cooked well on both sides, remove from heat and place on a plate with a paper towel to drain. Continue process until all tacos have been fried.
Garnish with lettuce or other leafy green, tomato, and salsa or hot sauce.