- 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.