How can I substitute for unsweetened chocolate in a frosting? I've found a recipe for cupcakes that I want to try. The ingredient list of the frosting is this: 


3 ounces (90 grams) unsweetened chocolate, coarsely chopped
1/2 cup (113 grams) unsalted butter, room temperature
1 cup (120 grams) confectioners (powdered or icing) sugar, sifted
1 teaspoon pure vanilla extract


However, where I live, unsweetened chocolate isn't available. I do have dark chocolate (cacao percentage is about 40, but I don't know how much sugar is in them).

I believe the frosting would be too sweet (even for a frosting) if I would use my chocolate. A simple solution seems to put in less sugar, but I think I'm going to end up with a different consistency.

So, how can I make the frosting so that it comes close to the original?
I'm willing to experiment (if for instance there would be a way to split the sugar from the chocolate).

Note: I have cacao powder at home, but I rather not use it for the frosting. The cupcakes themselves contain it and the combination of the two types of chocolate seem nice to me.
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:


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.
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!