In a saucepan, bring the water, sugar and corn syrup to a boil. Clean the sides of the pan with a wet brush to remove any sugar crystals. Cook without stirring until the mixture begins to caramelize. Add the peanuts and cook for 1 minute, stirring with a wooden spoon. Add the baking soda and stir to blend.
Pour the mixture onto the baking sheet and spread quickly with a spatula. Cool completely. Break half the mixture into chunks and chop the other half with a knife. Garnish the chocolate cake with the chunks of brittle while sprinkling the chopped peanut brittle in between the cake’s layers.