Why is my cake crumbly?
This looks very similar to a recipe that I make for chocolate cake. Mine was always a bit crumbly until I changed my measuring technique for the flour. Instead of scooping and leveling I spooned the flour in to the cup and then I leveled it. That made a huge difference. Of course, weighing is even better. 

It could also be an issue of slightly over baking. You could try testing them 5 minutes earlier. 

Those two things tend to be the crumbly culprits in cake.