Add the butter and spinach to a large skillet over medium heat. Saute the spinach until wilted, about 2-3 minutes.
In a large bowl, combine the spinach, ricotta, mozzarella, parmesan, pesto and whisked egg. Set aside.
Bring a large pot of salted water to a boil, and cook the pasta according to package directions until al dente, then strain the pasta.
Meanwhile, make the vodka sauce. Using the same skillet, add 1 tablespoon butter and onion. Cook over medium heat until the onion begins to soften, about 5 minutes. Add the garlic, red pepper flakes, oregano, and basil; cook for another minute.
Add the tomato paste to the skillet and continue to cook for 2-3 minutes, stirring occasionally. While mixing, slowly pour in the vodka and let cook off for about 1-2 minutes. Reduce to medium-low heat and add the heavy cream and salt whisking to combine. Let simmer for 2-3 minutes.
Mix in the remaining 4 tablespoons of butter and parmesan cheese.
Preheat the oven to 400 degrees.
Stuff each shell with about 1 tablespoon of cheese filling, then place in the skillet with the vodka sauce. Top with the shredded mozzarella and bake for 20-25 minutes, or until the cheese is melted and the sauce is bubbling around the edges.