1. Cream butter and sugars together until light and fluffy, about 3-4 minutes.
2. In a very small bowl, combine the hot water and instant espresso and stir until the espresso is dissolved. Mixture will be thick.
3. Add the espresso mixture to the butter mixture and mix to combine.
4. Add the egg and vanilla extract and mix until well combined.
5. In a separate bowl, combine the flour, baking soda and salt. Add the dry ingredients to the butter mixture and mix until well combined.
6. Stir in the chocolate chunks.
7. Make balls of cookie dough about 2 tablespoons in size. Refrigerate the dough for about 2 hours, or for up to 3 days.
8. Preheat oven to 350°F (176°C) and line a cookie sheet with a silicone baking mat or parchment paper.
9. Place the balls of cookie dough onto the cookie sheet and bake for 8-10 minutes. Cookies will spread a bit and may look a little undercooked when you remove them, but they’ll continue to cook and firm as they cool.
10. Remove the cookies from the oven and allow to cool on the cookie sheet for 3-5 minutes, then remove to a cooling rack to finish cooling