I'd leave it out. Those brownies are still suuuuper good without the espresso. It might be ok with the powder, but I'd be afraid that it'd be gritty.
Is there any liquid in the recipe? I can't remember, but if there is, maybe you could brew some espresso to use in place of the liquid.
ETA: duh, you already said the brewing thing. yes. do that.
So I just realized I don't actually have a way to brew the espresso. Oh well. Good to know they are good without it! I'm going to go ahead and make them without and just save these fancy illy grounds for the freezer and a rainy day ;)