Adding chocolate to peanut brittle makes the brittle taste different from any you've ever had before. Lightly butter a cookie sheet and set aside. In a small bowl, stir together the cocoa and baking soda then add the butter. Set aside. In a heavy 2 quart saucepan, stir together the sugar, corn syrup, and whipping cream. Cook, over medium heat, stirring constantly until the sugar is dissolved. Stir in the peanuts. Continue cooking, stirring frequently, until the mixture reaches 300 Degrees F. or when syrup dropped into very cold water separates into threads which are hard and brittle. (Make sure that the bulb of the candy thermometer is not resting on the bottom of the pan when using one.) Remove from the heat and stir in the cocoa mixture. Immediately pour onto the prepared cookie sheet. With tongs or wooden spoons, quickly spread and pull into a 1/4-inch thickness. Place the cookie sheet on a wire rack to cool completely. When the candy is cold, snap into pieces and store in a tightly covered container.