Put a rack in the middle of the oven and preheat oven to 350°F.
Melt butter in a baking sheet with sides in the oven.
Stir in rosemary, salt and cayenne pepper in a small bowl. Add to butter.
Toss walnuts in butter until well coated and spread out into one layer.
Bake for 10 minutes.
Serve warm or at room temperature.
Notes
* Store walnuts in refrigerator for a few weeks or freeze for a few months. * Never leave nuts cooked in butter or oil out at room temperature for more than a week as they may become rancid.