In a medium skillet combine sunflower kernels, pumpkin seeds, and sesame seeds. Cook and stir over medium heat for 3 to 5 minutes or until seeds are toasted. Remove seeds from skillet; cool.
In a medium mixing bowl beat butter with an electric mixer on medium to high speed about 30 seconds. Add 1/2 cup powdered sugar and vanilla, beating until combined and scraping sides of bowl occasionally. Beat in as much of the flour as you can with the mixer. With a wooden spoon stir in any remaining flour. Stir in toasted seeds. If dough is too soft or sticky, cover and chill for 1 hour or until dough is easy to handle.
Preheat oven to 350 degrees F. Shape dough in 2-1/2-inch-long ropes, then shape in crescents. Place 2 inches apart on an ungreased cookie sheet. Bake for 12 to 15 minutes or until bottoms are lightly browned. Cool on cookie sheet for 1 minute. Transfer to wire rack and cool completely. In a plastic bag gently shake cooled cookies, a few at a time, with the 1 cup powdered sugar. Makes about 36 cookies.