Preheat oven to 350 degrees and grease a 18x13 sheet cake pan. In a mixing bowl, combine flour, sugar and salt. Melt butter in a saucepan, add cocoa, stir together then add boiling water. Allow mixture to boil for 30 seconds, then turn off heat. Pour over flour mixture and stir lightly to cool. In a measuring cup, whisk together buttermilk, beaten eggs, baking soda and vanilla. Add to butter/chocolate mixture. Pour into sheet cake pan and bake for 20 minutes. Cool before frosting. While cake is baking, place chopped chocolate into a medium bowl and heat cream in a small saucepan over medium heat. When cream has come to a boil, remove from heat and pour over chopped chocolate. Whisk until smooth and allow to cool slightly before pouring over cake. Heat peanut butter in microwave for 30-40 seconds, until it can be poured but not until completely liquefied. Pour ganache and peanut butter across cooled cake making alternating stripes.
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