Dissolve espresso powder in boiling water; set aside to cool to tepid.
Place butter, confectioners' sugar, vanilla and dissolved espresso in a large bowl. Beat on medium speed until the mixture is smooth, about 1 minute. Do not beat until light and fluffy.
Add the flour and mix on low speed only until the flour disappears into the dough, about 30 seconds. Don't work the dough much once the flour is incorporated. Fold in the mini-chocolate chips with a wooden spoon.
Transfer the dough to a gallon-size Ziploc bag. Place the bag on a flat surface. Leaving a small opening in the top for the air to escape, roll the dough into a 1/4-inch thick rectangle that completely fills the bag. As you roll, turn the bag occasionally and lift the plastic from the dough so that it doesn't cause creases. (See Note 2)
Seal the bag and refrigerate the dough for at least 2 hours, or for up to 2 days.
Preheat the oven to 325° F. Line two baking sheets with parchment paper or silicone mats.
Place the Ziploc bag on a flat surface; cut away from the dough and discard bag. Turn the firm dough onto a parchment lined surface. Using a ruler as a guide and a pizza wheel or sharp knife, cut the dough into 1 1/2-inch squares. Transfer the squares to prepared baking sheets, leaving about 1/2-inch between the cookies. Carefully prick each one twice with a fork, gently pushing the tines through the cookies until they hit the baking sheet. Refrigerate for 20 minutes.
Bake at 325° F for 18 to 20 minutes, or until the edges of the cookies just start to lightly brown.
Transfer the cookies to a rack to cool completely. Store in airtight container for up to a week, if they last that long.
Yield: 4 dozen cookies.