Preheat the oven to 350°F and line a small 8x8 inch baking pan with parchment paper.
In a microwave-safe bowl, melt the white chocolate and butter together in 20-second bursts, stirring until smooth.
Stir the sugar into the melted white chocolate mixture until fully combined.
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
Fold in the flour and salt until no dry streaks remain.
Pour the batter into the prepared pan and spread it evenly.
Melt the dark chocolate in a separate bowl and drizzle it over the batter in a spiral pattern. Use a toothpick to drag lines from the center outward to create the spiderweb design.
Bake for 20-22 minutes until the edges are set and a toothpick comes out with a few moist crumbs.
Cool completely in the pan before cutting into squares.