A little update from yesterdays brewday.
I added the powder to recipe as suggested above. My pre-boil gravity was exactly what Beersmith3 told it would be. Nice!

So, I did the boil and added the powder, had a little taste of the wort and there was no flavor of the mint-cocoa powder (should have expected that...). So I decided to add some more at the end of the boil. First I was supposed to add only 1.4oz but ended up with 3.5oz. After chill the original gravity before pitching the yeast was 1.052 as it was supposed to be 1.062. Well, I pitched the yeast anyway and the beer is in full fermentation right now and everything is fine.
Later I decided to check out, what if I would have not added the powder to the recipe and see what the OG would look like. Turns out the recipe without the powder and sugars from it would be... drum roll, please... 1.052!! So as I was told that presenting that amount of powders sugars on that size of a batch it would not change drastically.
Thanks to everyone who helped me with this situation. Now I know that the next time I will not add it in the recipe as fermentable sugar. Oh, and I still did not get any mint-cocoa flavor so I think that I will add that one to the secondary fermantation.