Preheat oven to 350 degree F. Spray half sheet pan (size approximately 18x13-inches, I use this one from Amazon) with baking spray. Set aside.
For the cake, in a large bowl, combine the sugar, cocoa, flour, baking powder, baking soda and salt. Mix until blended.
Add in the eggs, oil, buttermilk, and hot water. Whisk together until fully combined. Pour into prepared baking sheet.
Bake for 25 minutes, until edges pull away from the pan and a toothpick inserted into the center comes out clean. Remove from oven and cool.
While cake is cooling, prepare chocolate icing. In a small saucepan, combine the butter, buttermilk, and cocoa powder. Whisk until smooth over medium high heat. Bring to a boil, and remove from heat as soon as it boils.
Pour chocolate mixture into a mixing bowl and add in the powdered sugar. Beat until smooth. Pour over cake (it's okay if the cake isn't completely cooled). Add sprinkles.
Cool completely and enjoy!
Video
Notes
To freeze, once the icing has hardened and set, and the cake is completely cooled, slice into portions. Using an airtight container, place pieces of cake in layers separated by wax paper. Freeze. When ready to enjoy, remove from freezer and allow to thaw at room temperature.