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.