Combine butternut squash, vegetable broth, potatoes, onion, carrots, and garlic in a 5-quart pot. Cover and cook over medium-high heat until the diced squash is easily pierced with a fork, about 45 minutes. Remove from heat.
Use an immersion blender to blend soup thoroughly in the pot. Add coconut milk. Heat over medium-high heat until warmed through, about 5 minutes. Remove from heat and season with salt and pepper.