A large-batch 9×13-inch pan brownie with shiny crackly tops, ultra-fudgy centers, and deep chocolate flavor from cocoa, melted chocolate, and chocolate chips.
Line a 9×13-inch light-colored metal baking pan with parchment paper, leaving overhang for easy removal.
Melt the butter and chopped semi-sweet baking chocolate together until smooth; keep warm.
With an electric mixer, whip the granulated sugar, brown sugar, 4 eggs, and 1 additional egg yolk until pale, thick, and at ribbon stage.
Mix in the warm melted chocolate-butter mixture, then beat in the vegetable oil and vanilla extract until combined.
Sift together the flour, Dutch-process cocoa powder, and salt; gently fold into the batter until just combined.
Fold in the chocolate chips (and nuts, if using).
Spread the thick batter evenly into the prepared pan.
Bake until a toothpick inserted in the center comes out with moist crumbs (not wet batter), about 30–32 minutes.
Cool completely in the pan, then lift out using the parchment and slice.