Preheat the oven to 325°F. Grease an 8x12-inch baking pan and line with parchment paper. Set aside.
Place the butter and 1 cup of the chocolate chunk in a medium glass bowl, melt together by placing in the microwave for 60 seconds. Stir the mixture, and then place in the microwave for an additional 30-60 seconds to melt completely.
Whisk in sugar, brown sugar, eggs and vanilla extract.
Add cassava flour, cocoa powder and Kosher salt. Stir together until combined.
Add ½ cup of the remaining chocolate chunks, and stir until combined.
Spread in the greased pan, and sprinkle the brownie batter with the remaining chocolate chunks and Maldon salt.
Place in the oven and bake for 25-30 minutes. Remove from the oven, and let cool for 5-10 minutes. Place the pan in the refrigerator on a cooling rack to cool completely, about 2 hours. NOTE: This step is critical, as it stops the baking and leaves you with gooey, fudgy brownies!
Cut into 18 equal pieces before serving.