Preheat the oven to 350°F and line two large baking sheets with parchment paper. Make the espresso by combining the espresso powder in a small bowl with the boiling water. If you’re using an espresso machine, make the espresso and measure out 1½ Tablespoons of espresso. Set aside.
In the bowl of a standing mixer or handheld mixer, beat butter, brown sugar, and granulated sugar until light in color and fluffy (around 1-2 minutes). Add in the egg, liquid espresso, and vanilla extract and mix until combined.
Add in the flour, salt, and baking soda and mix just until the last specks of flour disappear. Stir in the chocolate chips.
Use a cookie scoop or your hands to make 1.5 Tablespoon-sized balls of dough. Place them on the baking sheet, around 3-4 inches apart. Add a couple more chocolate chips on top if desired, then bake one sheet at a time for 8-11 minutes, until the edges look set and the middles look slightly underdone.
If the cookies have puffed up too much, gently but firmly tap the baking sheet against the counter right away after removing them from the oven. If desired, finely chop or shave a chocolate bar and sprinkle some chocolate in the middle of each cookie while they are warm.
Let the cookies cool on the baking sheet for 10 minutes then transfer to a cooling rack to cool completely.