Preheat oven to 350 and generously grease 2 large baking sheets.

Cook brown sugar, butter, and corn syrup in a saucepan over moderate heat, stirring occasionally, until smooth.

Remove from heat, then stir in flour and pecans.

Spoon scant tablespoons of batter 6 inches apart on baking sheets (4 cookies per sheet) and bake 1 sheet at a time in middle of oven until golden brown and bubbling, 6 to 8 minutes. (Cookies will spread to 5 to 6 inches.) 

Cool cookies on sheet 1 to 2 minutes.

Quickly but carefully remove cookies from sheet with a metal spatula and transfer to a rack to cool completely. (If they become too brittle to transfer, return baking sheet to oven for about 1 minute to allow cookies to soften.)

Be sure to cool these cookies before transferring them to a rack, they are very soft when they first come out of the oven and need to set a moment.