Preheat the oven to 350°F.

Line a 9x9-inch baking pan with parchment paper. Set aside.

Melt the butter in a medium mixing bowl using the microwave - should only take about 30 seconds or less.

Stir in the sugar until well incorporated.

Whisk in the eggs and vanilla until the mixture is smooth.

Fold in the cocoa powder, flour, and salt, mixing just until combined. Set aside.

Combine the white chocolate and butter in a separate medium mixing bowl. Microwave on 50% power in 30-second intervals, stirring in between each. It should only take about 30-40 seconds. Stir until smooth. Let cool slightly.

Whisk in the sugar, eggs, and vanilla until smooth.

Fold in the flour and salt, mixing just until combined.

Alternating between the two batters, add scoops of each into the prepared pan.

Use a knife or skewer to gently swirl the batters together, leaving visible streaks of each.

Bake for 35-40 minutes, or until the center is set and a toothpick inserted comes out with a few moist crumbs.

Cool completely before slicing to ensure clean edges and defined swirls.