Combine hot water and powdered sugar, mixing until completely smooth. Let it sit until it is just warm to the touch.
Using a whisk attachment, whip the butter until it is pale and doubled in fluffiness.
Switch to the paddle attachment. Slowly stream in the sugar and water glaze while mixing.
Add 4 to 5 tbsp of room temperature heavy cream and a pinch of salt to the bowl.
Scrape down the sides, then mix on the lowest setting using the 'low and slow' method to remove air pockets.
Test for doneness by gliding a spoon across the surface - it should be smooth with no air bubbles and show 3 to 4 ripples at the top.
