Cube the butter into 1 inch cubes

In a double boiler, heat the water, meringue powder, and salt and whisk until foamy. Add the sugar and whisk until dissolved

Beat the sugar mixture on medium high speed using a stand mixer, until glossy peaks form

Reduce the speed to medium and add the butter, one cube at a time. Add the vanilla and mix.

Increase the speed to medium high or high and beat for 2 minutes