Put the cubed potatoes into a large pot, and cover with cold water. Bring to a boil over medium-high heat and cook until the potatoes are tender, about 20 minutes. Drain and allow to cool. Once cooled, mash the potatoes with a fork and combine with salt and cornstarch and mix thoroughly. Set aside while ground beef is prepared.
While potatoes are cooking, make the ground beef. Heat a large skillet over medium high heat. Add the ground beef and break up into small pieces with a wooden spoon or spatula as it cooks. Cook, stirring occasionally, for 7-10 minutes, until beef is browned and cooked through. Drain and discard any excess grease/fat. Add tomato paste, spices and water and stir to combine. Allow to simmer for about 5 minutes, until beef has absorbed the moisture from the water. Set aside to cool.
To make avocado crema, combine avocado, cilantro, lime juice, cumin, garlic, salt and water in a high powdered blender or food processor. Blend to combine. Add more water to thin out the sauce if needed.
To assemble the croquettes, scoop out a golf ball size piece of potato dough, about 2 tablespoons worth, and flatten with the palms of your hands. Make a well in the center and add one tablespoon of the ground beef. Wrap the dough around the beef and lightly flatten it into a disk shaped patty. Set aside on a baking sheet. Repeat with remaining potato dough and ground beef.
Add 1-2 cups of oil to a heavy bottomed pot or high sided skillet. You'll want there to be about 1/2 inch of oil in the pan so adjust the amount of oil accordingly. Heat the oil to 350F. Add 3-4 potato croquettes to the skillet at a time, being sure not to crowd the skillet. Fry for 2 minutes on each side and carefully remove with a slotted spoon or spider skimmer (my fave!)
Serve warm with the avocado crema, some chopped cilantro and a squeeze of lime juice.