Preheat oven to 450°.
Combine butter and lemon rind in a small bowl; set aside.
Place a 12-cup muffin pan in oven for 3 minutes.
While pan heats, lightly spoon flour into a dry measuring cup; level with a knife. Process flour and next 4 ingredients in a blender 1 minute.
Remove pan from oven; coat with cooking spray. Pour batter evenly into cups, filling about half full.
Bake at 450° for 15 minutes. Reduce oven temperature to 350°, and bake 20 minutes or until puffed and brown. Serve immediately with lemon butter.