Preheat the oven to 375 degrees. Butter 4 ramekins and coat with 2 tablespoons of the Parmesan cheese.
In a medium saucepan over medium heat, melt butter. Stir in flour to make a paste. Gradually whisk in cream and bring mixture to a boil. Continue cooking for up to 3 minutes until thickened, whisking constantly. Transfer to a large bowl and allow to cool. Stir in egg yolks, sherry, cheese, sour cream, salt, Dijon mustard, dry mustard, cayenne and the remaining 2 tablespoons of Parmesan.
Place egg whites in separate bowl. Add cream of tartar. Using electric mixer, beat egg whites until firm peaks form. Fold one-third of whites into the soufflé base to lighten it. Then carefully fold in the remaining egg whites until no streaks remain.
Pour mixture into the ramekins. Run your thumb around the inside of the rim of the dish to wipe away any crumbs. I like to smooth the top of the batter.
Bake 25 - 35 minutes, until soufflé is golden brown and puffed. Serve right away.