Hi Mary, we checked with the author of this post, PJ, and luckily she just recently did some testing with adding a peanut butter swirl to brownies. She said she didn't add either an egg or sugar when swirling it into the brownies (though she did use a honey roasted peanut butter, so there was some sweetness in it already). She noticed that the peanut butter seemed to rise to the top of the brownies during baking, so you might experience something similar with the banana bread batter too. We think the marshmallow fluff would disappear/melt into the batter during baking, so you might want to save this ingredient for slathering on top of slices after they're done baking. As PJ said, give it a whirl (swirl!) and see what happens. We think it'll be a delicious experiment. Happy baking! Kye@KAF

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