Preheat oven to 350 degrees. Prepare your sheet pan/jelly roll pan with parchment paper or non-stick spray.
Combine butter, water, and cocoa in a medium/large saucepan on the stovetop and bring to boil. Once it boils, remove from heat and stir in vanilla extract.
In a large mixing bowl or stand mixer, combine all purpose flour, granulated sugar, baking soda and salt.
Add saucepan ingredients to dry ingredients, then mix lightly.
Add buttermilk or sour cream and eggs, mixing until no lumps remain.
Pour into prepared pan and place in preheated oven. Bake 20-22 minutes or until toothpick in center comes out clean.
Start preparing the frosting when cake has about 10 minutes to go.
Chocolate Frosting:
Combine butter and cocoa in a saucepan on top of the stove. Once boiling, remove from heat, add in vanilla and buttermilk/milk.
Place powdered sugar in large bowl or stand mixer and pour hot cocoa mixture on top.
Blend together until no lumps and frosting is smooth.
Spread frosting over cake while it is still hot and make sure to hit the entire top of the cake. Slice while warm or cool. Enjoy!