>> source: https://www.youtube.com/watch?v=D53Hf_WNJqc

Add @egg whites{4%large} and @granulated sugar{300%g} to a #double boiler or bain-marie{} on low heat and #whisk{} until all sugar is dissolved. Check by rubbing the mixture between your fingers.

Whisk in #stand mixer{} on medium high speed for about ~{10%minutes} until the mixture is completely cooled and fluffy.

Add @soft unsalted butter, cubed{400%g} to mixer on medium speed one cube at a time. Do not rush this step.

Add 1 tsp of @vanilla{1%tsp}.