Add espresso powder and hot water to a small bowl and stir until a paste forms. Set aside.
In the bowl of a stand mixer with paddle attachment or in a large bowl with a hand mixer, cream together butter both sugars until light and fluffy, about 2-3 minutes.
Add egg, vanilla, and espresso paste and mix until combined.
In a separate large bowl, whisk together flour, baking soda, and salt.
Mix the dry ingredients into the wet ingredients until just combined, taking care not to over-mix. Fold in chocolate chunks or chips, if using.
Put dough in fridge to chill for 30-60 minutes.
When cookie dough is almost done chilling, preheat oven to 350 degrees and line a baking sheet with parchment paper.
Scoop about 2 tablespoon of cookie dough (I use a #40 cookie scoop) onto the prepared baking sheets about 1 ½ inches apart. Top with more chocolate chunks if you want. Sprinkle tops of cookie dough with sea salt if desired. Bake cookies for 12-14 minutes, until cookies are just turning golden around the edges, but are still soft. Let cookies cool in the pan for a couple minutes before moving to a wire rack to cool completely.