Preheat oven to 400°F. Line a large baking sheet with aluminum foil. Place squash halves, cut side up, on prepared baking sheet. Brush with melted butter, and sprinkle with black pepper and 3/4 teaspoon of the salt. Roast squash in preheated oven until tender, 45 to 50 minutes. Remove squash from oven, and let stand until cool enough to handle. Scoop flesh into a medium bowl, and set aside.

Heat a large Dutch oven or heavy-bottomed saucepan over medium. Add pancetta; cook, stirring occasionally, until browned and crisp, 6 to 8 minutes. Transfer pancetta to a plate lined with paper towels to drain. Do not wipe Dutch oven clean.

Add onion to Dutch oven, and cook over medium, stirring often, until softened and fragrant, about 6 minutes. Stir in cayenne pepper and remaining 1 teaspoon salt; cook, stirring constantly, 30 seconds. Stir in chicken broth, thyme sprigs, and reserved squash. Bring to a boil over medium-high. Reduce heat to medium-low, and simmer, stirring occasionally, until flavors have melded, about 15 minutes.

Discard thyme sprigs. Pour half of squash mixture into a blender. Secure lid on blender, and remove center piece to allow steam to escape. Place a clean towel over opening. Process until smooth, about 1 minute. Repeat procedure with remaining squash mixture. Return soup to Dutch oven. Stir in cream.

Divide soup evenly among 6 bowls. Top each serving with a drizzle of heavy cream and a sprinkle of crispy pancetta. Garnish with thyme leaves.