In a stand mixer fitted with a paddle attachment, place the chilled melted brown butter, remaining 1/2 cup butter and cream cheese. Beat the mixture on high speed until creamy and smooth. Scrape down the sides of the bowl a couple of times. Beat in the vanilla and salt. With the machine running on low speed, add 1 cup of icing sugar and then increase the speed to high and beat it well. Add the remaining icing sugar in batches of 1 cup, 1 cup, 1/2 cup and then a final 1/2 cup if needed, stopping to scrape down the sides of the bowl a few times. I use about 4 cups of icing sugar, but if it’s thick and creamy after 31/2 cups, stop then. If you feel like you need more icing sugar to make a thick and creamy icing, then by all means add more icing sugar.