A few weeks ago I made a European-ish buttercream with roughly these ingredients:
4 large whole eggs plus two yolks; ~1.5 cups of white sugar; ~1T vanilla bean paste (I scraped out the rest of my jar); 5 sticks of salted butter (I like salted butter. Haters gonna hate.)
I beat it over a double-boiler until it was foamy, cooking it to at least 140F. Then I transferred it over to the stand-mixer, beat it until it was cool to the touch, added the butter bit by bit, and continued to beat it until soft and fluffy. It was perfect for my affogato cupcakes.
Half an hour ago, I made a lemon-polenta loaf, but I don't like powdered sugar glazes, so I thought I'd beat a bunch of lemon juice and lemon juice powder into some of the left-over vanilla buttercream living in my fridge. I did that, and the texture is weirdly reminiscent of mayonnaise. Not bad, just noticeable.
And when I thought about it, I realised it has all the necessary components. Eggs, oil (in this case just butter), and acid (about a tablespoon each of King Arthur lemon juice powder and fresh lemon juice). The eggs and oil were already emulsified, which seems to be the hardest part of mayonnaise-making, and I just beat the acid into my preexisting emulsion. Except for the vanilla and lots of sugar, it fits the definition (I think).
So my question is, could I, in fact have turned my fancy buttercream into mayonnaise? Or is this just a trick of texture? Is there any way I can tell for sure with a little I save out? If it's not mayonnaise, then why not?
Corollary-type question: If this is mayonnaise, would there be anyway to make a tart European-ish buttercream without it turning into mayonnaise...?