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:


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!