Can anyone help me?

I am doing a bridal shower cake and the customer requested the Cake Boss White Velvet recipe (https://www.cakeboss.com/WhiteVelvetCake.aspx).  The cake is due this Sunday and she just requested that one of the layers be chocolate.  I have never altered a recipe, so I'm not sure exactly how to achieve this successfully.  Do I use cocoa powder?  How much?  Will adding powder dry out the cake?  Since this was a last minute request, I am under strict time constraints, (I also have a full time job), so I don't have time to experiment with the cake to get it right.

The woman hosting the shower is also getting married this year and whether or not she hires me to do her wedding cake hinges on the success of this shower cake.  Thank you!!!

I've never used this recipe or doctored a cake mix, but couldn't you substitute a chocolate cake mix and then proceed with the recipe?

Were you planning to mix a separate batch of batter?  Then use a chocolate cake mix, and 1/3 of the flour should be cocoa powder.

Were going to make a large batch of batter? Then you will have to add water as well as cocoa powder.  How much depends on the pan and amount of batter. I mix cocoa powder and boiling water to a soft paste, let that cool, and add to white batter for marble cake.

Most bakers have a cutoff point in their contracts.  After the cutoff, either no changes are allowed, or a surcharge will apply.  Sounds like you will need a written contract with this additional clause. 

AOh duh! I am so stressed about things right now I didn't even consider that I could just use a chocolate mix in place of the white mix the recipe calls for. I'm such a dummy! As you can probably tell I'm new to this... Thank you anyway for your time and advice on such a silly question!