Butter is typically added when the fudge is first taken off the heat- but it isn't mixed in. The butter is allowed to melt across the surface to keep it from forming a skin on top.
Vanilla, nuts, and all other additions are mixed in at the end of the cooling period when the fudge is stirred.
Alton Brown, with some help from Shirley Corriher, explained the process well.
Looking at your recipe, the most obvious thing to me is that there is no salt. Adding a small quantity of salt (say, 1/2 tsp) will enhance the flavors of the ingredients already present.
The second thing you might try is switching to dutch processed cocoa; many people find this has a more intense chocolaty taste.
You could try enhancing the overall flavor by adding a small amount of cinnamon (say 1/4 tsp) or instant espresso powder (perhaps 1 tbl). While these ingredients do not, in small amounts, overwhelm the flavor, they do increase its complexity and the impression of how chocolaty the fudge is.
Of course, you can increase the amount of cocoa powder (at least by ratio): simply increase the absolute amount of cocoa, starting with small increments of perhaps 1 tablespoon per test run. The problem with this method is not only that it throws off the sweetness balance, but also that it will eventually change the chemistry, possibly influencing the crystallization of the sugar phase which is what provides the smoothness of the fudge.
Finally, and more radically, consider reducing the amount of dairy, changing the milk for water, or reducing the amount of butter. The milk fats and milk solids tend to mask the flavor of chocolate. While I consider these part of the overall desirable balance of the fudge, since you after an intense chocolate experience, it may be worth experimenting with.
Best Answer
There are recipes that specifically use condensed and or evaporated milk as a shortcut to making fudge. It's not just a drop in replacement, it's a whole change in method. Just do a search for "easy fudge recipe" on Google to get examples of these fudge recipes that use condensed milk.
To add my opinion however, the old fashioned cooking and churning method produces the best fudge, but the condensed milk method although isn't as good at least produces a more consistent result. I do the old fashioned method. It's worth the extra effort and the extra learning curve (to recognize when the fudge is about to setup).