Preheat oven 350 degrees F. In bowl, whisking flour, baking soda, sugar and salt and set apart
In 10 inch cast iron skillet, bring butter, vegetable oil, cocoa powder also water to boil.
Remove from the heat and mix in the dry ingredients well. Combine buttermilk, egg and vanilla. Bake the cake in the pan at 350 degrees F for about 15 to 20 minutes or until a toothpick comes out with just a few moist crumbs.
While the cake is starting to cool, prepare the frosting. In saucepan, bring the butter, cocoa also milk to boil. Remove them from the heat and add the icing sugar, walnuts and vanilla. Stir to combine.
Pour over warm cake, spread with spatula and serve with vanilla bean ice cream, caramel sauce and whipped cream.

Enjoy