1. To make the baked tostada shells, preheat the oven to 400 degrees F. Spray each side of the corn tortillas with cooking spray. Season with salt. Place tortillas on a large baking sheet and bake until tostadas are lightly browned and crisp on one side, about 3-5 minutes. Remove from oven and flip the tortillas. Place back in the oven and cook until lightly browned and crisp on the other side, about 3-5 minutes. Set aside.
2. In a large bowl, add the butternut squash, olive oil, cumin, lime juice, salt, and pepper. Toss until squash is well coated. Spread the squash out in a single layer on a large baking sheet. Place in the 400 degree F oven and roast until the squash is tender and starts to caramelize on the edges, about 30-35 minutes.
3. While the squash is roasting, make the creamy avocado dressing. Combine the avocados, buttermilk, garlic, cilantro, green onion, jalapeño, lime juice, cumin, salt, and pepper in a blender or food processor and blend until smooth.
4. To assemble the tostadas, top a crispy corn tortilla with butternut squash, black beans, red onion, lettuce, and pepitas. Drizzle the creamy avocado dressing on top. Serve immediately.
Note-if you can't find blue corn tortillas, use yellow or white corn tortillas. Also, feel free to add your favorite toppings to the tostadas: cheese, salsa, black olives, etc.