Preheat oven to 200°F and line a large baking sheet with a piece of parchment paper or a Silpat® baking mat. Set aside.
Divide 1½ cups of the shredded cheese into 6 piles (approximately 1/4 cup each) on the prepared baking sheet and spread each pile into a thin circular layer. Place baking sheets in the preheated oven until the cheese is melted and slightly browned around the edges, around 5-6 minutes.
Remove from the oven and let cool for 2-3 minutes, then remove the cheese and press into 3” ramekins or a muffin tin to form cups. Let cool for several minutes before gently removing the cheese cups from the form.
Note: If using ramekins, the taco cups can remain in the container for extra support when serving.
In the meantime, add the ground beef to a large skillet over medium heat and cook until the beef is browned and cooked through, approximately 6-8 minutes. Carefully drain excess grease from skillet and return to the cooktop.
While the beef is cooking, combine the avocado, tomato, lime juice, and fresh cilantro in a medium bowl. Season with salt and black pepper, to taste, and gently stir to combine.
To serve, fill the taco cheese cups with beef, avocado salsa, and remaining shredded cheese. Top with a dollop of sour cream, if using, and enjoy!