why do my devils food cake has a hard top when I take it out of oven please help

You may be over baking it. 

AThat and make sure your oven is the correct temperature, a too-hot oven can cause it too. You can a buy an oven thermometer pretty cheap if you don't have one. My oven bakes 25 degrees higher than it's set at!

Also, make sure your oven racks are in the middle of the oven. My oven cooks 25 degrees too hot too!!! I guess all ovens now are made in China and they can not tell how to calibrate oven temperature!!! We no longer have quality control since everything now made in China and overseas!!!

Sugar not being mixed/dissolved properly can cause the surface of the cake to become hard.  Do you make your Devil's Food using the put chocolate, sugar, butter etc etc in a saucepan and heating it till it dissolves and becomes liquid chocolate mix which you then add to dry ingredients?  If so, it's probably not been heated and mixed enough for the sugar to properly dissolve into the chocolate etc.

ATry using a light colored aluminium pan. Dark colored pans absorb more heat and absorb fast too. This results in a overbaked/hard cake. You can also try baking your cake in a rack below the middle rack. Helps me :)