Start by preheating your oven to 375°F (190°C). This step is essential as it ensures that your cookies bake evenly and achieve the right texture.

In a large bowl, cream together the softened butter, dark brown sugar, and granulated sugar until smooth. This should take about 2-3 minutes. Then, add the egg and vanilla extract. Mix until blended well.

In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, sea salt, espresso powder, and cocoa powder. This mixture helps incorporate all the dry elements evenly, ensuring a consistent texture.

Gradually mix the dry ingredients into the wet mixture. Stir until just combined. Avoid over-mixing; you want to keep that tender texture.

Now, fold in the chocolate chips and chopped walnuts. The goal is to distribute them evenly throughout the cookie dough.

Drop tablespoonfuls of dough onto a baking sheet lined with parchment paper. Leave enough space between each cookie to allow for spreading. Bake in the preheated oven for about 10-12 minutes or until the edges are lightly golden. The centers may look slightly underbaked, but don’t worry, they'll firm up as they cool.

Once baked, take them out and let them rest on the baking sheet for a few minutes before transferring them to a wire rack. Allow them to cool completely (if you can resist!) and enjoy the warm, gooey deliciousness.