Preheat oven to 350°F
Grease your pan(s)
In a saucepan, melt butter + water + cocoa, whisking until smooth
Remove from heat
In a large bowl, whisk together sugar, flour, salt, and baking soda
Pour the warm cocoa mixture over the dry ingredients and stir until combined
Add sour cream, eggs, and vanilla
Beat 30–45 seconds, just until smooth
Pour into prepared pan(s) and bake for 22–28 minutes (9x13 pan), 18–22 minutes (13x18 sheet pan), 22–26 minutes (two 8-inch round pans), or 18–24 minutes (two 9-inch round pans)
Cool completely before frosting
In a saucepan, melt butter + milk + cocoa, whisking until smooth
Remove from heat
Whisk in powdered sugar + vanilla until glossy and smooth
Spread frosting immediately
Slice and serve
