Preheat your oven to 350°F (175°C). Grease a 9 by 13 inch baking pan with butter or nonstick spray. Dust it lightly with flour and tap out the excess.

In a large bowl, whisk together the flour, sugar, and baking soda. This ensures the leavening is evenly distributed.

In a medium saucepan, melt the butter over medium heat. Stir in the cocoa powder until smooth. Add the water, buttermilk, and beaten eggs. Stir constantly as the mixture heats. You want it to come to a boil, but do not walk away. The eggs can scramble if left unattended. Once it boils, remove the pan from the heat immediately.

Pour the hot chocolate mixture into the bowl with the flour mixture. Stir until the batter is smooth and no dry streaks remain. Mix in the vanilla extract.

Pour the batter into the prepared pan and spread it evenly. Bake for 20 to 25 minutes. A toothpick inserted into the center should come out clean. The top will look set and spring back lightly when touched.

Let the cake cool in the pan on a wire rack for 10 minutes. Drop spoonfuls of peanut butter over the warm cake. Wait a minute for it to soften, then spread it gently in an even layer. The warmth of the cake helps the peanut butter melt into a glossy layer. Allow the cake to cool completely.

Place the sifted confectioners' sugar in a large bowl. In a small saucepan, combine the butter, buttermilk, and cocoa. Cook over medium heat, stirring constantly, until the mixture begins to boil. Pour the hot mixture over the sugar and stir until smooth. Add the vanilla and mix well.

Pour the icing over the peanut butter layer. Spread it carefully to the edges. The icing will begin to set as it cools. Serve immediately for a gooey texture, or let it sit at room temperature until the icing firms up.