Preheat the oven to 325 degrees Fahrenheit. In a small bowl, combine melted butter, garlic salt, and 1/4 teaspoon chili powder. Using a 3-inch biscuit or cookie cutter, cut circles from the tortillas—each tortilla should yield 3 to 4 circles. Stack the circles, wrap them in a damp paper towel, and microwave for 20 seconds to soften. Brush both sides of each tortilla circle with the butter mixture, then press them into the cups of a mini muffin pan to form the shape of the cups. Bake for 15 to 18 minutes. Let them cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

In a large nonstick skillet over medium-high heat, cook the ground beef, breaking it up with a wooden spoon. Once most of the pink is gone, add the diced onion and cook for an additional 2 minutes. Stir in chili powder, cumin, and cayenne pepper, cooking for 30 seconds before adding the tomato sauce. Simmer until the sauce thickens, then season with salt and pepper to taste.

In a small microwave-safe bowl, combine the refried beans and shredded cheese. Heat for 30 seconds, stirring, and continue heating in 20-second intervals until the cheese is fully melted.

Spoon a small amount of the bean and cheese mixture into the bottom of each baked taco shell, then add a spoonful of the meat mixture on top. Garnish with shredded lettuce, diced tomato, red onion, avocado, and cilantro. Finish with a small dollop of sour cream.