Heat butter in a medium sauce pan over medium-high heat.
Add onion and saute until translucent, about 10 minutes.
Add mushrooms and vodka and saute for 5 minutes more, or until vodka has evaporated.
Stir in broth, heavy cream and sauce. Cook, stirring occasionally, for 10 minutes; add basil, and season with salt and pepper to taste.
To serve, top soup with seasoned croutons.