Combine butter, corn syrup, sugar, peanut butter and vanilla in a large bowl and beat at medium speed until smooth. At low speed, gradually beat in flour until well blended.
If necessary, chill 1 hour or until easy to handle.
Preheat oven to 350°F. To form letters, roll dough into 1-1/2-inch balls; on smooth surface, roll dough into 8-inch long ropes. Shape ropes into desired letter directly on ungreased cookie sheet.
Bake 12 to 15 minutes or until lightly browned. Remove from cookie sheet; cool on rack. May be stored in tightly covered container up to 3 weeks.