01 -
Preheat oven to 175°C (350°F). Line an 8×8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 -
In a microwave-safe bowl, melt the butter, vegetable oil, and chopped bittersweet chocolate in 30-second intervals, stirring until smooth.
03 -
Whisk the espresso powder and Dutch-process cocoa powder into the melted chocolate mixture to bloom the cocoa and intensify flavor. Set aside to cool slightly.
04 -
In a separate large bowl, whisk granulated sugar, powdered sugar, and vanilla extract until thick, glossy, and lighter in color (about 3 to 6 minutes using a stand or hand mixer).
05 -
Beat in the whole eggs and additional egg yolk to the sugar mixture until fully incorporated.
06 -
Slowly fold the cooled chocolate mixture into the egg-sugar mixture until homogeneous.
07 -
Sift together flour, cornstarch, and salt. Gently fold them into the batter using a rubber spatula, taking care not to overmix.
08 -
Carefully fold in semi-sweet chocolate chips and toasted nuts if using.
09 -
Pour batter into the prepared pan, smoothing the surface. Bake for 23 to 25 minutes until edges are set and a toothpick inserted in the center comes out with moist crumbs.
10 -
Allow brownies to cool completely in the pan. For cleaner slicing, chill in the refrigerator briefly and wipe the knife between cuts. Serve plain or with ice cream.