Dissolve the gelatin in hot water.
Begin whipping the heavy cream with a whisk attachment on your mixer. While mixing slowly pour in the gelatin mixture. (It should still be warm enough to pour as a liquid but not extremely hot. If it is too cool it will gel up in your whipped cream instead of blending in.)
Beat to stiff peaks and set aside.
In the bowl of a stand mixer fitted with the paddle attachment or with a high powered hand mixer, beat the butter on medium speed until it is smooth, about 1 minute. (This step is incredibly important for a smooth frosting. I skipped it which is why my frosting is a bit lumpy.)
Add the sweeteners and cocoa powder to the bowl and beat until combined.
With the mixer running on low speed, slowly stream in the milk and vanilla extract then add the salt and continue beating until well combined, scraping down the sides as needed, about 2 minutes.
Increase the speed to high and beat the frosting for an additional 2 minutes. Add in the whipped cream mixture and mix until fully incorporated.
Use the frosting immediately or store in an airtight container in the fridge. (If you refrigerate the frosting, you may want to re-blend it for a few seconds in the mixer before using it.)