Add brown sugar and water to a small pot with a candy thermometer. The hot syrup should be brought to a softball stage that is 240°F.  Once it reaches 240°F remove it from the stove and beginning pouring in the meringue in a steady stream.  Be careful of the hot syrup.

While the syrup is boiling separate the eggs and using the whisk attachment mix until the meringue reaches stiff peaks.  You know when its stiff peak with the meringue is glossy and points straight up when you pick up the whisk and does not collapse at all.

Once the hot syrup reaches 240°F turn the mixer on high, pour into the meringue, and beat until the mixing bowl is cool to the touch.  Depending on how much buttercream frosting you are making this can range from 20 minutes to 35 minutes.

When the bowl is cool to the touch begin adding the butter in small pieces one at a time. The lovely meringue you just made will deflate a little and look like you did something absolutely wrong.   Just keep mixing until it comes together.

The buttercream should pass the upside-down spatula test.  Place some buttercream on a spatula then turn it upside down, if it does not fall off the spatula you are good to go.  If it slides off you will have to mix some more.