Preheat oven to 350º F and grease a 18×13 sheet cake pan.
In a mixing bowl, combine flour, sugar and salt.
Melt butter in a sauce pan, 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 sauce pan 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 liquified.
Pour ganache and peanut butter across cooled cake making alternating stripes, (think: American flag!). Swirl with a butter knife or similar utensil to create marbling effect.
FUN TIP: To create an easy pattern, drag a toothpick perpendicularly through the stripes of ganache and peanut butter!