Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a heatproof bowl, combine the finely chopped semi-sweet chocolate and butter. Place the bowl over a pot of simmering water and stir until melted and smooth. Remove from heat and cool slightly.
In a separate bowl, whisk together the all-purpose flour, Dutch-process cocoa powder, baking powder, salt, and espresso powder.
In a large mixing bowl, combine the light brown sugar and granulated sugar. Pour the melted chocolate mixture into the sugars, whisking until smooth. Add the vanilla extract and mix well.
Crack the eggs into the chocolate mixture, one at a time, mixing well after each addition.
Gradually add the dry ingredient mixture to the wet ingredients, stirring gently until just combined.
Gently fold in the semi-sweet chocolate chips, chocolate chunks, and finely chopped walnuts.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheet, spacing them about two inches apart. Bake for 10-12 minutes, until the edges are set but the centers remain slightly soft.
Remove from the oven and allow to cool on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely.