Have the butter, heavy cream, salt and vanilla ready as well as a heat proof jar or bowl.
Add the sugar to a 2 quart pot and turn the heat to medium. You can add a squeeze of lemon and stir it in, it helps prevent crystallization. Let it first melt, then as you see it begin to caramelize, stir with a wooden spoon until all of the sugar is melted and fully caramelized.
Turn the heat down and add the heavy cream slowly while stirring (take care as it will boil up). Add all of the butter and stir until smooth. Turn off the heat and add the salt and vanilla.
Pour the caramel into the heatproof jar or bowl and let it cool just for a few minutes, so it’s not too hot going onto the brownie.
Spread the caramel over the brownies in an even layer and let chill until the caramel is mostly firm (about an hour).
Lay out a cookie sheet lined with parchment paper and slice the brownies. If the caramel is at room temperature and not too firm it will slide onto the sides of the brownie to give more of a coating.