Take the saucepan off from the stove. Remove lemongrass stalk from the broth. Add about one-third of the roasted butternut pumpkin cubes to saucepan. Puree soup with an immersion blender. Set the saucepan back to heat and add rest of the broth, lime juice, tamarind paste, soy sauce, brown sugar, and coconut milk. Adjust the taste. Add rest of the chili slices if desired. Add more broth if the soup seems too thick. Cube the tofu and add it, along with the remaining pumpkin, to the soup.