How can I substitute for unsweetened chocolate in a frosting?

The reason this frosting uses unsweetened chocolate is because with all the sugar and sweetened chocolate (even dark, ie 70%) it would become cloyingly sweet. It needs to have double the amount of icing sugar to butter as this produces a smooth texture. If you don't have enough icing sugar it won't have a smooth texture and will have a hard texture almost like the state of the butter to begin with. So there are 2 ways of getting around this:

1. This is the easiest method. You can use the original recipe substituting unsweetened chocolate for dark chocolate (preferably with minimum cocoa solids of 65% to keep the chocolatey flavour) and add a quarter of a cup of cocoa powder to balance out the sweetness. To equate for the stiffness it brings slowly add 3 or 4 tbs's of milk so it has a smooth frosting-like texture.

2. Make an meringue frosting. This is appropriate because the meringue keeps the buttercream smooth and light without adding huge amounts of sugar, so you can then use your chosen chocolate and not have it too sweet. To do this you have to make an Italian meringue which is hot sugar syrup whipped into a meringue (or a Swiss meringue but this uses a different method which involves whipping sugar and egg whites until hot in bowl over a pan of water) then once cool whipping in soft chunks of butter. Then you can add your melted cooled chocolate. Here are some recipes for Italian meringue buttercream (US measurements), Italian meringue buttercream (metric measurements), Swiss meringue buttercream (US measurements) and finally Swiss meringue buttercream (metric measurements).

Hope this helps!

I would halve the amount of icing sugar and substitute in cocoa powder. As long as you sift the cocoa powder well you should still have a smooth icing at the end. Provided your chocolate isn't milk chocolate you should still have a 'proper' chocolate flavour as well.

I would also use cocoa powder (not sweetened chocolate powder) in place of some of the icing sugar. It would help you maintain the consistency you are after, though the exact quantities you would want to use would need to be adjusted to suit the chocolate you are using. The bitterness of the cocoa powder will help balance the sweetness of the sugar and the chocolate.

Dark chocolate should be unsweetened chocolate plus sugar - those ingredients will account for most of the mass. Your 40% cacao bar is therefore 40g chocolate to 60g sugar (stabilizers or emulsifiers like soy lecithin should be negligible in volume, just a trace amount percentage wise).

So, if you had 200g of the dark chocolate, that would be 80g of unsweetened chocolate, and 120g of sugar... within reasonable reach of your recipe. Maybe add a few extra grams since the chocolate is slightly less, and the sugar won't have cornstarch added like powdered sugar usually does (3% by weight, I think). Since the unsweetened chocolate is chopped, I'm assuming it gets melted in the process of making the frosting - so it might not change the texture too much if the icing was going to be mixed with hot chocolate anyway.

In any case, to make the icing you would be melting the chocolate and when it's cool enough (the texture will change if the butter melts), mixing it with the room temperature butter to make a thick chocolate sauce - kinda comparable to ganache, but no cream. You won't be able to cream the butter with the sugar, but you can probably beat the whole chocolate/butter mixture (with the vanilla) until it's fluffier. Or beat a bit of cocoa powder in with the butter (texturally, it should help, and balance the slightly less chocolate).

Of course, the flavor profile will be eerily similar to your chocolate bar - just a bit more buttery and with a dash of vanilla flavor, so only take this route if you're happy with that bar's flavor.

Alternately, you could try using half as much dark chocolate, and using a combination of powdered sugar and cocoa powder for the other half of what the recipe calls for. So, 100g dark chocolate with 60g sugar and 50g cocoa powder. This will let you cream the butter and sugar for texture, and the extra solids in the cocoa powder should be balanced with the extra cocoa butter from the melted dark chocolate so it doesn't make it too stiff. This might get you something closer to your original recipe, working with the ingredients you have.