I used this recipe on smitten kitchen as a starting point, making just a few changes, mostly to make it nut-safe. These are outlandishly good. They have a strong coffee flavor, but my kids still liked them.
1 1/2 tablespoons instant coffee granules (I used decaf since that's what was in the house)
1 tablespoon boiling water
1 cup unsalted butter, softened
2/3 cup confectioners' sugar
1/2 teaspoon vanilla
2 cups all-purpose flour
4 ounces chocolate chips (eyeball it based on the size of your bag...I have an 8 oz bag so used about half of it)
Dissolve the coffee in the boiling water and set aside to cool.
Put about 1/4 of your chocolate chips on a cutting board and with a large knife, cut the chips into smaller pieces. You'll end up with some piece that are practically shaved, and some rather large pieces. Transfer to another container and continue with the rest of the chips until you have chopped them all. If you're not worried about nut allergies*, you could just use mini chips and skip this, though I really liked how the different sized pieces of chocolate tasted in the cookies.
Beat the butter and sugar on medium speed until smooth and well-blended. Beat in the vanilla and coffee. Reduce the blender speed and mix in the flour until just incorporated. With a rubber spatula, fold in the chopped chocolate chips.
The dough will be very sticky. Transfer it to a gallon sized ziplock bag. Flatten the dough slightly with your hands pushing on the outside of the bag, then use a rolling pin to roll the dough out until it fills pretty much all the bag. You'll need to leave the bag unzipped while you do this, and you'll have to stop occasionally to unstick the plastic from the dough to remove trapped air or straighten out the bag so you don't get creases. It works pretty great, though, and the dough should be about 1/4" thick when you're done. Seal the bag and place it on a flat surface in the fridge for at least 2 hours.
Preheat the oven to 325 degrees.
Line a cookie sheet with parchment paper. Cut the plastic bag along the seams to expose the dough. Using a pizza cutter, cut the cookies into squares (whatever size you'd like...mine were about 1 1/2" square, or at least that's what I was aiming for). Place cookies on baking sheet. They don't really expand when cooking, so you only need to leave about 1" between cookies. Prick each cookie twice with a fork, pushing all the way through the cookie until you touch the tray. (Try to avoid any larger pieces of chocolate when you do this...I had trouble a few times with the tines of the fork getting stuck in a big glob of chocolate!)
Bake for 18-20 minutes until very lightly browned. Cool for a few minutes on the tray, then transfer to a cooling rack.
*Note about nut allergies: it's hard to find chocolate in the supermarket that is nut-safe. I've had the most luck finding standard sized chocolate chips that are nut-safe. Trader Joe's are my favorite, though as always with allergies, check the bag to make sure they're safe as manufacturing processes can change.