In a mixing bowl, whisk together all frosting ingredients. Pour over brownies in an even layer, then place in the freezer on a flat surface. Sprinkle on extra sea salt and cacao nibs, if using.
Freeze for at least 30 minutes before removing and slicing. I personally don't cover the dish for these 30 minutes but you can! Brownies will keep tightly sealed in the refrigerator for up to 1 week or in the freezer 2 months. Enjoy!