Preheat oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper, leaving an overhang and lightly greasing it.

Melt 1 cup chopped dark chocolate and ½ cup unsalted butter in a microwave, stirring every 30 seconds until smooth, then let it cool slightly.

Whisk 1 cup granulated sugar and ½ cup packed light brown sugar into the cooled chocolate mixture, then beat in 4 eggs one at a time and stir in 1 teaspoon vanilla extract.

In a separate bowl, whisk together 1 cup all-purpose flour, ¼ cup unsweetened cocoa powder, and ½ teaspoon salt; gradually fold into the wet ingredients until just combined, being careful not to overmix.

Melt ½ cup unsalted butter in a small saucepan over medium heat, stir in ½ cup granulated sugar and ¼ teaspoon salt, cooking for 2-3 minutes until sugar dissolves and mixture bubbles.

Remove from heat and stir in ½ cup heavy cream and 1 teaspoon vanilla extract.

Pour two-thirds of the brownie batter into the prepared pan, spread evenly, and drizzle half of the butterscotch sauce over it.

Dollop the remaining brownie batter over the butterscotch, then drizzle the remaining butterscotch sauce on top.

Gently swirl the butterscotch into the brownie batter with a knife or skewer for a marbled pattern.

Bake for 28-35 minutes, until edges are set and a toothpick inserted into the center comes out with moist crumbs.

Cool completely in the pan on a wire rack before lifting out with parchment paper and cutting into squares.