Melt butter in heavy large pot over medium-high heat. Add onion, tomatillos and garlic and saute' 5 minutes. Mix in 4 cups corn, 3 cups stock, peas, and cilantro sprigs. Puree mixture in blender in batches. Return puree to pot and bring to simmer. Add chilies, spinach, sugar, remaining corn and remaining 1 cup stock to soup. Simmer 15 minutes. Season to taste with salt. (Can be prepared 1 day ahead. Cover and refrigerate. Bring to simmer before continuing.)
Ladle soup into bowls. Sprinkle with tortilla chips; top with sour cream and chopped cilantro.