Preheat the oven to 375.
Whip butter with mixer until fluffy. Stir in the powdered sugar, cornstarch, and flour. Beat on low speed for 1 min., then on high for 3 - 4 min. Drop by spoonfuls onto ungreased cookie sheet.
Beat for 12 - 15 min. Watch that edges don't brown too much. Cool on racks.
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