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.