Preheat oven to 375°F. Lightly mist muffin tin with cooking spray.
Place a damp paper towel on a microwave-safe plate. Working three at a time, cover tortillas with a second damp paper towel and microwave for 10 seconds on High power. Tortillas will be Hot!
Working quickly, form tortillas into a bowl in the bottom of muffin wells, overlapping the sides, or turn muffin pan upside down and place tortillas between cups to make a shallow bowl.
Bake in preheated oven for 10 - 12 minutes, until crisp and starting to brown. Tortillas will continue to crisp as they cool. Leave oven on.
To Make the Filling:
In a large skillet over medium-high heat, brown the beef until no pink remains. Use wooden spoon to break up chunks.
Add taco seasoning and 2/3 cup water. Stir to combine.
Bring beef mixture to a boil, then reduce heat and simmer for 8 -10 minutes until liquid has evaporated.
Divide meat evenly among prepared shells, using spoon to pack down. Sprinkle with cheese.
Return filled shells to oven and bake 3 - 4 minutes until cheese has melted.