Why would a recipe call for pouring boiling butter into egg-whites?
This is from a low-carb blog, specifically a gluten-free fluff marshmallow frosting recipe with that specific instruction – I asked the writer why she does it that way and here is her response:


  There are a few reasons why the hot butter is poured into the beaten egg
  whites. First of all, the egg whites are beaten first to give the
  maximum volume to the frosting. Secondly, the butter mixture is poured
  while it’s hot to prevent gelatin forming lumps or erythritol
  crystallizing. The hot butter also gives some shine to the frosting
  when poured into the beaten egg whites.


This is step five on her list of directions; her response is at the very bottom of the page in the comment section. 

http://www.lowcarbsosimple.com/low-carb-marshmallow-fluff-frosting/#comment-276