Preheat your oven to 350 degrees and grease an 8 inch square cake pan. Optionally line the pan with parchment paper and set aside. In a large bowl, whisk together the flour, sugar, baking soda, and salt. In a microwave safe medium bowl, melt the butter until just melted in the microwave, about 2 minutes. Then add in the water, and microwave for another minute, or until the mixture is just boiling. Stir in the cocoa powder and whisk to combine until smooth.

Pour the chocolate mixture over the dry ingredients, and whisk until combined- the mixture will be very thick at this point. Then stir in the buttermilk, egg, and vanilla extract and whisk until combined and smooth. Pour the batter into your prepared cake pan, and bake for 30-35 minutes, or until a toothpick comes out with just a few crumbs clinging to it. Cool the cake completely before frosting it.

Melt the chocolate in the microwave, stirring every 30 seconds, until it is completely melted, about 1-2 minutes, and set this aside to cool. In a medium bowl, beat together the butter and peanut butter until smooth. Add in the powdered sugar slowly, and begin to mix until it starts to combine. The mixture will be very thick and crumbly at this stage. Mix in the salt, vanilla extract, and 2 tbs. of the heavy cream until combined. Then beat in the melted chocolate until mixed through. If the frosting is still too thick, beat in the remaining heavy cream until just combined, and the frosting becomes thick. Frost your cooled cake and enjoy!