Put the sausages in a small skillet with 3/4 inches of water. Bring to a boil over medium heat and par-cook the sausages for 7 to 8 minutes.
Meanwhile, warm the stock and saffron in a medium pot over low heat.
Heat a large heavy pot or deep skillet, over medium-high heat and add the extra-virgin olive oil. When the oil smokes, add the chicken and season liberally with salt and pepper, to taste. Brown the chicken for a couple minutes on each side, then remove it to a plate. Add the par-boiled sausage and crisp up the casings, 2 to 3 minutes. Remove the sausages and cut them into large chunks. Add the butter to the pot and let it melt.
Add the pasta to the melted butter and let it toast for a couple minutes, then add the onions and garlic. Season the pasta with salt and pepper, to taste, then add the bay leaf and stir for 2 to 3 minutes more. Add the rice and stir to combine, then deglaze the pot with the white wine, stirring for 30 seconds. Put the chicken and sausages back into the pot, pour in the saffron stock and stir.
Cover the pot with a tight-fitting lid and simmer for 15 minutes. Add the seafood and tomatoes and cook until the shrimp are opaque, about 3 to 4 minutes more. If using mussels, cook until the mussels are open. Discard any that do not open. Remove the bay and discard. Transfer the paella to a large serving platter, top with basil, and serve.