Lightly butter a cookie sheet; set aside. In small bowl, stir together cocoa and baking soda; add butter. Set aside.

In heavy 2-quart saucepan, stir together sugar, corn syrup and whipping cream. Cook over medium heat, stirring constantly until sugar is dissolved. Stir in peanuts. Continue cooking, stirring frequently, until mixture reaches 300°F on candy thermometer or until syrup, when dropped into very cold water, separates into threads which are hard and brittle. (Bulb of candy thermometer should not rest on bottom of saucepan.)

Remove from heat; stir in cocoa mixture. Immediately pour onto prepared cookie sheet. With tongs or wooden spoons, quickly spread and pull into 1/4-inch thickness. Place cookie sheet on wire rack; cool completely. Snap into pieces; store in tightly covered container.