Abbey, this looks wonderful and sounds like it was completely delicious!
re: subbing chocolate of different cacao percentages, it works well in recipes where the chocolate is melted and blended with other ingredients and there’s a way to incorporate the sugar so that it dissolves and works well in the recipe. I’m not sure with this frosting, because the chocolate is in chunks, if it would be as successful. Probably it would as the chunks are so tiny. I might wonder if you would get a chunk and it would be too bitter when you crunched it. But if they are tiny enough, they would probably always be with enough of the sweeter frosting base that it would be fine.
Anyway, the formula goes like this:
need 100g of 60% chocolate
have 85% chocolate in pantry
the amount of chocolate liquor you need is 100g x 60% or 60 grams
the amount of sugar you need is about 40% of 100g or 40 grams.
to get 60 grams of cacao with 85% chocolate, 60 / 0.85 = 70.6 grams of 85% chocolate (round up to 71 grams unless your scale is accurate to tenths or something like that)
So you’ll need 71 grams of 85% chocolate to get the same 60 grams of cacao that is in 100grams of 60% chocolate.
Now for the sugar:
to find out how much sugar you have in 71 grams of 85% chocolate, 71 x 0.15 = 11 grams of sugar in 71 grams of 85% chocolate.
you have 11 grams but need 40 grams, so 40-11 = 29 grams of additional sugar needs to be added to the recipe.
This is a bit simplified because sugar and cacao/chocolate liquor aren’t the only two ingredients in chocolate (lecithin and sometimes vanilla are other ingredients), but it is close enough and I’ve used it with success many, many times.