Why would a recipe call for pouring boiling butter into egg-whites? I made a sponge cake using a recipe from an old cookbook and at one point it told me to melt and boil butter and pour the boiling butter into beaten egg-whites, let sit for a minute and then mix gently. This procedure puzzles me. Why should I pour in the butter while hot?
If you want the whole thing, here goes (no measures, 'cause it's in Polish and uses Polish measures, where &quot;one cup&quot; is not what you would call &quot;one cup&quot;):

beat egg whites with powdered sugar until stiff
pour in melted and boiling butter (or shortening), mix gently after one minute and let sit until it cools.
Start mixing again and add egg yolks one by one, then lemon juice and finally flour mixed with baking powder.
Mix a bit more.
Bake for at least 50 minutes at 160 degrees C

The batter looks fine, it's baking nicely, but the long time is also strange for me, most sponge cakes that I made require just 20-25 minutes of baking. I skimmed the book, but didn't notice any other sponge cake recipe with this approach.
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