Bring salted water to a boil in a large pot. Add the pasta and cook it according to the package directions until al dente, about 8-10 minutes.
While the pasta is cooking, cook the shrimp in a large pan, heat the butter and olive oil over low heat. Add the garlic, red pepper flakes, salt and black pepper and cook about 4 minutes, stirring occasionally, until the garlic is tender but not browned.
Increase heat to medium and add shrimp and sautué, turning the shrimp once with tongs, until shrimp are just cooked through, about 2 minutes each side. Set aside.
Make the avocado sauce: Place the avocado in a blender or food processor, add the remaining sauce ingredients and process until smooth and creamy.
When pasta is done cooking, drain and place pasta into a large bowl. Add the avocado sauce to the pasta and toss until pasta is well coated.
Using tongs, add the shrimp to the pasta and pour the garlic sauce over the shrimp. Garnish with parsley or cilantro and serve immediately.