Bring sugars and milk to a boil in a Dutch oven, stirring often. Cook over medium heat, stirring often, 11 minutes or until a candy thermometer registers 228° (thread stage).
Stir in butter and pecans; cook, stirring constantly, until candy thermometer registers 236° (soft ball stage).
Remove from heat; stir in vanilla. Beat with a wooden spoon 1 to 2 minutes or just until mixture begins to thicken. Quickly drop by heaping tablespoonfuls onto buttered wax paper or parchment paper; let stand until firm.