Ensure the rinsed spinach is patted completely dry.
Add the spinach, dried basil, garlic, walnuts, and olive oil to a high-speed blender or food processor.
Blend until the mixture is smooth, pausing to scrape down the sides of the container as necessary to ensure everything is incorporated.
Pour the blended pesto into a bowl.
Stir in the grated Parmesan cheese by hand until just combined.
Gently mix in the softened butter and salt until the pesto is creamy and well-blended.
Taste and adjust seasoning if necessary before serving.