combine butter, brown sugar, and vanilla. beat at medium speed, scraping bowl often until creamy (1-2 minutes). add flour, reduce speed to low. beat until mixture leaves sides of the bowl and forms a smooth, soft dough (2-5 minutes).
divide dough in half. shape each half into a 2 inch round log. wrap each log in plastic wrap. refridgerate for at least 2 hours.
heat oven to 375ºF. cut logs into 1/4 inch thick slices. place on ungreased cookie sheet. bake for 7-9 minutes or until lightly browned on the edges. let stand for 5 minutes before removing from cookie sheet.