Did I turn my frosting into mayonnaise? 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...?
It might have the consistency of mayonnaise but so do many sauces, sweet or savory. If it tastes good to you, it's fine so use it.

What I do when I want a tart citrus taste (for many things, not just icing) is add citric acid crystals. I consider citric acid to be essential in a well-stocked kitchen. If you'd like it to taste like lemon or orange, add some finely grated peel of which ever you prefer. I find a slightly tangy orange flavored icing goes wonderful with chocolate cake. 

I can't say how much citric acid to add as I rarely use a recipe. When you're adding butter and whipping the icing, I think that's the best time to add a little citric acid (and peel, if you wish). Start off with just 1/8 teaspoon and after incorporating it well, take a taste. Start adding it well before you've whipped in all the butter. Keep adding 1/8 teaspoonful at each time (or less when you think it's almost right). Keep track of how much you've added so you can write it down for next time. Use a little less if you include orange zest as oranges aren't meant to be as tangy as lemons. Personally, I like tangy taste to many foods.

I use salted butter in all my baking too and simply don't add any extra salt. I like salt. I've asked friends if they found it a little salty and no one said they did.