Using a 2-inch biscuit cutter, cut out the cooled brownies. Place them on a cooling rack inside a cookie sheet. (I like to eat the extra brownie pieces.)
Place the marshmallow buttercream in a piping bag fitted with a tip. (Or, use a plastic sandwich bag with a corner snipped off.)
On top of each cooled brownie, pipe the frosting in a circle, squeezing with even pressure straight overhead. Starting at the edge, pipe two layers, making sure to leave a hole in the center.
Put the caramel sauce in a piping bag (or plastic sandwich bag with the corner cut off). Pipe the caramel sauce into the center of the buttercream swirls, filling to the top.
Next, continue the buttercream swirl by piping a little more to cover the caramel center filling.
Chill the brownies in the freezer for about 5 minutes as you melt the chocolate.