Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
Melt the butter and half of the chocolate chunks in a microwave-safe bowl, stirring every 30 seconds until smooth.
In a large bowl, whisk together the sugar, eggs, and vanilla extract until well combined.
Add the melted chocolate mixture to the egg mixture and stir until combined.
Sift in the flour, cocoa powder, baking powder, and salt. Mix until just combined.
Fold in the remaining chocolate chunks and chopped walnuts.
Drop spoonfuls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes, or until the edges are set but the centers are still soft.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.