Combine pecans, sugar, milk, and syrup in a small Dutch oven; mix well. Slowly bring to a boil, stirring constantly, until sugar dissolves. Cover and cook 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches soft ball stage (234°).
Remove from heat, and add butter, without stirring. Let cool to room temperature. Add vanilla; beat with a wooden spoon until mixture is creamy and begins to thicken.
Working rapidly, drop mixture by rounded tablespoonfuls onto waxed paper. Let cool.