So there's Anchoïade (https://everydayfrenchchef.com/recipes/starters/anchoiade/) And then there's Anchoïade douce, which seems to get translated as sweet -- https://needl.co/app/p/jeanmartin/1547/sweet_anchovy_sauce and https://www.bipia.com/en/sauces-and-condiments-with-espelette-chili-pepper/118-antxo-pintxo-130-g-anchoiade-douce-aux-piments-3584384301146.html -- , but maybe just gentle or mild is better translation?

There's this Japanese recipe for fried, dried anchovies in coated with brown sugar and chillie powder. https://runawayspoon.wordpress.com/2009/12/11/spicy-sweet-anchovies-dilis/

But still, i can't imagine that in buttercream.