Preheat oven to 340°F (170°C) and line an 8x8 or 9x9 inch square baking pan with parchment paper leaving an overhang on two sides for easy lifting.

Melt chopped dark chocolate and butter together in a heatproof bowl over barely simmering water, stirring gently until completely smooth and glossy.

Remove melted chocolate mixture from heat and set aside to cool fully to room temperature — do not skip this cooling step.

In a large mixing bowl beat eggs and granulated sugar together on medium-high speed for 3–4 minutes until pale, thick, significantly increased in volume, and falling in thick ribbons from the beaters.

With mixer on low or using a spatula gently fold the fully cooled chocolate and butter mixture into the beaten eggs — use slow deliberate folding motions to avoid deflating the batter.

Sift scant 1 cup of flour directly into the chocolate batter, add vanilla extract, and fold gently until no dry flour streaks remain — stop folding immediately once flour is fully incorporated.

Pour approximately half of the brownie batter into the prepared pan and spread into an even layer.

Arrange white chocolate bar pieces in a single even layer across the entire surface of the batter covering it as thoroughly as possible.

Pour remaining half of brownie batter over the white chocolate layer and spread carefully and evenly to cover all white chocolate completely.

Bake on center rack at 340°F for 25–30 minutes — begin checking at 25 minutes.

Brownies are done when top is set and slightly crackled, edges are firm and pulling from pan sides, and a toothpick inserted in center comes out with moist fudgy crumbs — not wet batter and not clean.

Remove from oven and cool completely in the pan for 45–60 minutes before cutting — do not cut while warm.

Use parchment overhang to lift brownie slab onto a cutting board, slice into squares with a sharp knife wiping blade clean between each cut, and serve.