Preheat oven to 350F.
Add the beef to a skillet over medium heat. Brown for 5-6 minutes, until no pink remains. Drain fat.
Add 1/3 cup water to the skillet along with chili powder, cumin, garlic powder, salt and pepper. Stir to combine. Simmer for 2-3 minutes. Set aside.
Spray mini muffin tins with cooking spray.
Stack three corn tortillas and place on a paper towel or plate. Heat tortillas in microwave for 20 seconds. Remove from microwave and immediately cut three 3-inch circles out of each corn tortilla with either a cookie cutter or a glass. It's okay if they overlap a bit and you don't get exactly 3 inches for each circle.
Gently press in the muffin tin to get a cup shape. Repeat with remaining tortillas, working in batches.
Add a spoonful of ground beef to each muffin cup. Top with one teaspoon of shredded cheese.
Bake at 350F for 10 minutes, until cheese is melted and tortilla is crunchy. Let sit for 2-3 minutes in the muffin tin before removing carefully with a knife. Set on a paper towel for a minute to absorb all the excess oil and grease.
Top mini taco cups with diced tomatoes, sour cream, green onions and cilantro. Serve.