Combine whipping cream, sugar, syrup, and butter in a 3- quart saucepan; cook over low heat, stirring constantly, until sugar dissolves. Cook over medium heat, without stirring, until mixture registers 232° (thread stage) on a candy thermometer. Remove from heat.
Combine cereal and peanuts in a large mixing bowl. Pour hot sugar mixture over cereal mixture, stirring well to combine. Press firmly into a greased 13- x 9- x 2-inch baking pan, using back of spoon. Cool and cut into 2- x 1-inch bars.