Preheat your oven to 350°F (175°C). Line a 9x13 inch metal baking pan with parchment paper, allowing extra to hang over the edges.
In a medium saucepan, melt the butter over medium heat. Once melted, stir in the coconut sugar until fully combined. Set aside to cool slightly.
In a large mixing bowl, whisk the eggs and vanilla extract. Gradually incorporate the cooled butter-sugar mixture.
Gently fold in cocoa powder followed by the gluten-free flour blend, baking powder, and salt, mixing until just combined.
Stir in 6 ounces of the dark chocolate chunks. Pour the batter into the prepared pan and smooth the top.
Scatter the remaining 2 ounces of chocolate chunks over the top. Bake for 30-35 minutes or until a toothpick inserted in the center comes out with moist crumbs.
Allow the brownies to cool completely in the pan. Lift out the brownies using the parchment paper overhang, slice into squares, and serve.
Notes
For best results, use a high-quality unsweetened cocoa powder. Store any leftovers in an airtight container at room temperature for up to three days, or refrigerate for up to a week.