Line an 8-inch square baking pan with foil, extending foil over edges of pan. Generously coat foil with cooking spray; set aside.
Melt 1 cup butter in a 3-quart sauce-pan over low heat. Stir in brown sugar, condensed milk, and corn syrup until smooth. Bring mixture to a boil. Cook over medium heat, stirring often, until a candy thermometer registers 235°.
Remove mixture from heat; stir by hand 1 minute or until mixture is smooth and no longer bubbling. Quickly pour mixture into prepared pan; let stand 3 hours or until cool.
Lift foil and caramel out of pan. Cut caramels into 1-inch pieces with a buttered knife. Wrap each piece with plastic wrap.