Preheat oven to 275 degrees Fahrenheit.
Place sugars in food processor and process until very fine. Add butter; pulse until sugar disappears.
Add flour; pulse until no dry flour particles remain. (Mixture will be moist and crumbly.)
Place in plastic bag and press together to form dough; knead lightly until dough holds together. Remove from bag.
Shape dough into 1-inch balls. Place on unbuttered cookie sheets 2 inches apart.
Flatten each with a cookie press, fork, or the bottom of a glass lightly moistened with water. Bake 45 minutes to 1 hour or until pale golden. If using Silpat mats or something similar, may take as little as 30 minutes. WATCH CAREFULLY!
Remove from cookie sheets; cool completely on wire racks.