Preheat oven to 350-degrees. Grease jelly-roll pan and set aside.
Cake
Add the flour and sugar to a large bowl. Mix it up and set it aside.
Add the oil, butter, and water to a saucepan, turn it on medium-high, and bring it to a boil.
Pour the boiling oil mixture over the flour and mix until well blended.
Add the eggs, cinnamon, baking soda, vanilla, and buttermilk and mix with a beater until well blended.
Pour into the jelly-roll pan and bake for 20-25 minutes until cake tester comes out clean.
Allow cake to cool for at least 30 minutes.
Frosting
Add the butter and brown sugar to a sauce pan and cook on medium-high.
Bring the mixture to a boil and cook until it begins to thicken and the brown sugar dissolves.
Remove from heat and let it cool for a minute or two.
Add the milk and mix it in.
Add the powdered sugar and mix with a beater until it is smooth.
Pour over the cake and let cool.
