Then, in a large bowl, whisk together flour, baking soda, sugar and salt. Set aside.
In a 10-inch cast iron skillet, bring the butter, vegetable oil, cocoa powder, and water to a boil. Remove from heat and begin to whisk in the dry ingredients.
Next, with a spatula, incorporate the buttermilk, egg, and vanilla until just combined.
Bake the cake for 20 - 25 minutes or until a toothpick inserted near the center comes out clean.
While the cake begins to cool, prepare the icing. In a saucepan, bring the butter, cocoa, and milk to a boil. Remove the pan from the heat and stir in the peanut butter.
Next, add the powdered sugar and vanilla. Mix until smooth. Depending on the consistency, add a the milk, one tablespoon at a time, until the icing is just pourable but not runny. Pour over the warm cake and smooth out with a spatula.
Allow the icing to set before serving with vanilla ice cream.