Line the cookie sheet with parchment paper or aluminum foil. Set aside.
Melt the semi-sweet chocolate in the microwave. Heat for 2-3 minutes, mixing the chocolate in 30 second intervals. You can also use a double boiler method to melt the chocolate. Heat & stir until chocolate is fully melted.
Melt the white chocolate in the microwave. Heat for 1-2 minutes, mixing the chocolate in 30 second intervals. Heat & stir until chocolate is fully melted.
Spread the melted semi-sweet chocolate evenly throughout the cookie sheet.
Drizzle the melted white chocolate over the semi-sweet chocolate.
Use a knife or a toothpick to swirl the white chocolate into the semi-sweet chocolate.