Preheat the oven to 180°C (350°F).

Add the caster sugar, brown sugar, and egg to a mixing bowl.

Whisk the sugars and egg until the mixture is light, pale, and slightly thickened.

Place the dark chocolate and butter in a heatproof bowl.

Microwave the chocolate and butter in short bursts until melted, stirring between bursts.

Pour the melted chocolate mixture into the whisked egg mixture.

Stir the batter until the chocolate mixture is fully incorporated.

Add the all-purpose flour and cocoa powder to the batter.

Fold the dry ingredients into the batter just until no dry streaks remain.

Fold the chocolate chips into the brownie batter.

Spread about half of the brownie batter into the 6-inch oven-safe skillet.

Dollop about half of the pistachio spread over the brownie batter.

Cover the pistachio spread with the remaining brownie batter.

Dollop the remaining pistachio spread over the top of the batter.

Swirl the top pistachio spread through the batter with the silicone spatula.

Bake the skillet brownie for 15 to 20 minutes, until the edges are set and the center remains fudgy.

Cool the brownie skillet for at least 10 minutes before serving warm.