One more suggestion: regardless of how much buckwheat flour you add, be sure to let the dough rest for a good 15 minutes before you shape and bake them. As I mentioned, buckwheat flour is a large granule which takes some time to hydrate. When I make buckwheat crepes, for example, I make the batter the night before. Please let us know how they turn out!

I agree with drbabs and boulangere. I've done it, but adapting a recipe designed to be whole wheat choc chip cookies. I think I added 1/2 c (it was by weight, so keep that in mind). They turned out extremely soft, actually. More soft than I anticipated and not a bit grainy. I did add water, as per boulangere.

Hope that helps. If this is a traditional recipe with AP flour, i'd go no more than 1/4 c of total flour as drbabs suggested.

I agree with drbabs, but I'd go for even less, say a couple of tablespoons. Buckwheat flour is a great big granule, and needs large amounts of fluid to adequately hydrate it so it isn't crumbly and doesn't taste like ground cardboard. You may want to add a couple of tablespoons of water as well because it increases considerably in size.

Yes, I think you can, but I'd start with subbing no more than about 1/4 of the total flour. And your cookies may be denser and more crumbly because of the reduced gluten. Please let us know how they turn out.