A gentle braising ensures that squid is soft--not rubbery, as it can be if overcooked on higher heat. Rice is then cooked in the broth, the starch making the liquid creamy. Squid ink provides unusual color and a hint of pleasant briny flavor.
Bring stock and 4 cups water to a boil in a medium saucepan. Turn off heat.
Melt 1/4 cup butter in another medium saucepan over medium heat. Add 1/4 cup oil, half the onions, the garlic, and 1 tablespoon parsley. Cook, stirring occasionally, until onion is translucent, about 6 minutes. Add squid; cook, stirring, 1 minute. Add 1/2 cup wine; reduce heat to medium-low. Cook, stirring occasionally, until squid is tender, about 45 minutes.
Cook chorizo in a dry skillet over medium-low heat, stirring occasionally, until golden brown, about 4 minutes. Using a slotted spoon, transfer to a plate lined with paper towels.
Melt remaining 1/4 cup butter in a large saucepan over medium heat. Add remaining 1/4 cup oil and remaining onion; cook, stirring occasionally, until onion is translucent, about 4 minutes. Add rice, and cook, stirring constantly, until edges of grains are translucent, 2 to 3 minutes. Raise heat to medium-high. Add remaining 1/2 cup wine; cook, stirring constantly, until wine has evaporated.
Add squid and braising liquid; cook, stirring, until liquid has been absorbed. Add 4 cups stock, 1/2 cup at a time, stirring constantly, waiting for each addition to be absorbed before adding the next. Stir in squid ink after 20 minutes. Cook, adding 1/2 to 1 cup more stock in the same manner as above, until liquid is creamy and grains are cooked through but centers are still slightly firm (total time will be about 25 minutes).
Remove from heat; stir in remaining 2 tablespoons parsley and the salt. Serve topped with chorizo and more parsley.