Combine sugar, milk, syrup, and peanuts in a small Dutch oven. Slowly bring to a boil, stirring until sugar dissolves. Cover and continue to cook over medium heat 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. Add butter and vanilla (do not stir). Let cool to lukewarm (110°). Beat with a wooden spoon until mixture begins to thicken. Drop by tablespoonfuls onto waxed paper; let cool completely. Store in airtight containers in a cool place.