1. Preheat oven to 350°F.
2. Add the flour, sugar, cocoa powder, baking soda and salt to an ungreased 9×13 inch baking pan and gently whisk together to combine. Spread mixture evenly in pan.
3. Make three wells in the dry ingredients – one larger and two smaller. Add the oil to the larger well and the vanilla extract and vinegar to the other two wells..
4. Pour the water evenly over the entire pan of ingredients, then use a whisk or fork to gently combine everything together.
5. Bake for 25-30 minutes, or until a toothpick inserted in the middle comes out clean.
6. Allow cake to cool, then make the frosting.
7. For the frosting, add the butter, milk and cocoa powder to a small saucepan and cook over medium-low heat, stirring often, until the mixture comes to a simmer.
8. Remove the pan from the heat and whisk in the powdered sugar, vanilla extract and salt. Whisk until everything is nice and smooth, then immediately pour the frosting over the cake and spread into an even layer. The frosting starts to set quickly, so you want to work fast. If adding sprinkles on top, have them handy and ready to go, or they won’t stick to the frosting well once it begins to firm up.
9. Allow frosting to cool, then serve.