Heat olive oil in a large skillet. Add chilis, onion, and half the garlic and cook for about 5 minutes. Add tomatillos, one teaspoon of salt and agave or sugar. Cook mixture on medium-low for about 10 minutes, until tomatillos are soft. Remove from heat and puree tomatillo sauce in a blender, food processor or with an immersion blender. Meanwhile, melt butter in a skillet and add shrimp, half teaspoon of salt, pepper, hot pepper flakes, cumin and remaining garlic. Cook 3-4 minutes or until shrimp are completely pink. Add blended tomatillo sauce to the shrimp and simmer for 3-4 minutes. Serve hot over rice and sprinkle with fresh cilantro.