Cake with an impenetrable crust?
I blame the recipe, which calls for absurdly long mixing times, well past the point of overmixing.

After you add the wet ingredients, the recipe has you mix for:


30 seconds at low speed for the milk
30 seconds at medium speed for the milk
An unspecified amount of time at low speed to incorporate the first egg
2 minutes at medium speed for all the eggs (30 seconds after each)


That is simply way too much mixing for a cake.  Personally, I don't even like the concept of using an electric mixer at all, except to cream the butter.  But that's over 3 minutes of mixing and it is simply too high.  You'll end up with extremely tough, chewy cake, practically bread.

If you look at the top-rated from-scratch cake recipes on various recipe sites (e.g. Epicurious) you'll see that they all either call for incorporating the wet ingredients either by folding with a rubber spatula (best) or with an electric beater at low speed.

Beware of "manufacturer" recipes - I don't think I've ever had a positive experience with one.  You would think that they are authoritative sources, but in practice you'll achieve much better results with a search engine and careful reading of the reviews.