Preheat the oven to 350 degrees.

In a medium pot, combine butter, water, and cocoa and bring to a light boil. Whisk constantly until the mixture starts to bubble and becomes glossy. Remove from heat and set aside.

In a large bowl sift the sugar, flour, salt, and baking soda together.

Pour the cocoa mixture over the flour mixture and fold until almost combined completely.

Add buttermilk, sour cream, beaten eggs, and vanilla; mix until just combined.

Pour the batter into a greased 13×18 inch jelly roll pan.

Bake at 350 degrees for 20 to 25 minutes. Let cool as you prepare the frosting.

With an electric mixture or a stand mixture. Beat the butter and sugar, starting low, progressing to a medium high speed. Beat for 4 to 6 minutes or until the mixture is light and fluffy.

Stop the mixture and scrape down the sides of the bowl. Add the cocoa powder, vanilla, and salt,mix for 30 seconds.

Add in half of the heavy whipping cream and mix for 30 seconds add in the rest of the heavy whipping cream and increase the speed to medium-high. Whisk for 4-5 minutes or until the mixture is light and fluffy. If the cake is cool spread the mixture evenly over the cake.