In large measuring cup add oil, cocoa and water. Microwave 1 minute, set aside. In large mixing bowl combine flour, sugar soda and salt. Add buttermilk, eggs, vanilla and hot cocoa liquid. Mix until blended (mixture will be runny) Pour into a sheet pan and bake 20-25 minutes in 350 degree oven. While cake is baking, make icing: Bring butter, milk and cocoa to a boil. add vanilla and powdered sugar - mix until creamy - add pecans. When cake is done, remove and poke holes in top. Spread icing over hot cake so it melts into holes.