Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper.
Melt 4 ounces of the chocolate in the microwave in 20-second intervals. Set aside to cool.
8 ounces dark chocolate chunks
In a large bowl, beat the butter and sugar with an electric hand mixer until well combined. Add eggs and vanilla and beat for 1-2 minutes, until fluffy and light in color.
10 tablespoons unsalted butter, 1 cup granulated sugar, 2 large eggs, 2 teaspoons pure vanilla extract
Whisk in the melted chocolate, then sift in the flour, cocoa powder, and salt. Fold to incorporate the dry ingredients, being careful not to over mix as this will cause the brownies to be more cake-like in texture.
¾ cup all-purpose flour, ¼ cup unsweetened cocoa powder, ½ teaspoon kosher salt
Fold in the remaining chocolate chunks, then transfer the batter to the prepared baking pan.
Bake for 20-25 minutes, then remove to cool completely.