Preheat oven to 350°F (175°C).

Grease and line an 8-inch square baking tin with parchment paper.

Melt 1/4 cup butter and dark chocolate in a double boiler until smooth.

In a bowl, whisk 1/2 cup sugar, vanilla, and an egg until combined.

Stir in the melted dark chocolate mixture.

Add 1/2 cup flour and mix until just combined.

Melt 1/4 cup butter and white chocolate chips in a double boiler until smooth.

In a separate bowl, whisk 1/2 cup sugar, vanilla, and an egg until combined.

Stir in the melted white chocolate mixture.

Add 1/2 cup flour and mix until just combined.

Drop spoonfuls of each mixture into the prepared tin, alternating colors.

Gently smooth the surface without blending the mixtures.

Bake for 35 minutes, or until firm.

Cool completely in the tin before cutting into squares.