Add unsalted butter or more EVOO if using fat free Greek yogurt?

I’ve boosted fat content of non-fat yogurt back to full-fat level and it helps with both flavor profile and mouth-feel.
So, yes, go ahead.
Either the unsalted butter or more olive oil would work.
But since the amount is so small (adding 3.6 to 6ml or about a teaspoon), I would go for the olive oil as easier.
Your choice.
Recipe sounds delicious - i’ll add it to my “olive oil cakes” group!