Can I increase the cocoa content in fudge?
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.