Place your butter and shortening together in the bowl of your mixer. Mix them together on medium speed until they are nice and creamy.
Turn the mixer down to the lowest setting and slowly incorporate the powdered sugar into the shortening mixture.
Once you have added all the powdered sugar in to your mixture, add in the vanilla extract and then the hot water.
Turn the mixer up to medium-high speed and continue mixing for 10 minutes until light and fluffy.
The buttercream can be stored at room temperature for up to three days or in the refrigerator for up to two weeks. When you bring it out of the refrigerator let it sit at room temperature for two hours. Then re-whip it for ten minutes to bring it back to life.