This is the sugar cookie recipe I usually use:

http://allrecipes.com/Recipe/Thick-Cut-Outs/Detail.asp

I'd really like to make a chocolate variant. Do you think that substituting a cup of cocoa powder for a cup of the flour would work? Less? More?

Any advice much appreciated.

The link didn't work, for me. I have no advice right now, sorry.

Huh. It worked when I first posted it, but not now. Odd. Sorry!

At any rate, there are seven cups of flour in the recipe, 2 1/8 cups sugar, 1 lb. butter, etc, etc. I think replacing some of the flour with cocoa powder would work, but I'm undecided on how much.

As before ... any advice appreciated!

The link won't work for me, either.

This is a great recipe:
http://cakecentral.com/cake_recipe-6959-Dark-Chocolate-Rolled-Sugar-Cookies.html