Preheat your oven to 350°F (175°C) and prepare two baking sheets by lining them with parchment paper to prevent sticking.
In a large mixing bowl, use an electric mixer to cream together the softened butter, granulated sugar, and packed brown sugar. Mix on medium speed until the mixture becomes light, fluffy, and pale in color (approximately 2-3 minutes).
Add the eggs to the bowl one at a time, ensuring each is fully incorporated before adding the next. Follow this by mixing in the pure vanilla extract and espresso powder until the mixture is smooth and homogenous.
In a separate bowl, whisk together the sifted all-purpose flour, baking soda, and fine sea salt. Gradually incorporate this dry mixture into the wet ingredients, mixing on low speed just until combined. Be careful not to overmix to keep the cookies tender.
Using a spatula, gently fold in the semi-sweet chocolate chips and chopped walnuts (if you're including them) until they're evenly distributed throughout the dough.
Using a cookie scoop or a tablespoon, portion out rounded dollops of dough onto the prepared baking sheets, leaving enough space between each cookie to allow for expansion during baking.
Bake in the preheated oven for 10-12 minutes, or until the edges are a lovely golden brown and the centers appear slightly soft (they will firm up as they cool on the baking sheets).
Once baked, let the cookies cool on the baking sheets for about 5 minutes before carefully transferring them to a wire cooling rack to cool completely.
Notes
Serve warm with espresso or milk. Dust with powdered sugar for presentation.