Step 1

Preheat oven to 350 degrees. Brush a 13-by-18-inch rimmed baking sheet with butter. In a bowl, whisk together flour, cocoa, baking soda, and salt until thoroughly combined.

Step 2

In a large heatproof bowl set over a pot of simmering water, melt butter with 10 ounces (2 cups) bittersweet chocolate. Remove from heat; stir until smooth. Let cool 5 minutes. Add both sugars, eggs, and vanilla and beat on medium speed, or whisk vigorously by hand, until thickened slightly and glossy, 2 to 3 minutes. Reduce speed to low; beat in flour mixture just until combined. Stir in two-thirds of nuts and dried fruits.

Step 3

Transfer batter to prepared sheet, spreading evenly to edges. Sprinkle evenly with white chocolate and remaining nuts, dried fruits, and 1 cup bittersweet chocolate. Gently press toppings into batter with flattened palms.

Step 4

Bake until edges are puffed and just set but center is still wobbly, 20 to 23 minutes. Transfer pan to a wire rack; let cool completely. Cut lengthwise into thirds, then crosswise into 1-inch-thick rectangles. Bars can be stored in an airtight container at room temperature up to 5 days, or stacked between layers of parchment in a freezer bag and frozen up to 1 month. (Thaw at room temperature before serving.)